This document provides a framework in which the quality of a
dataset can be described, whether by the dataset publisher or by a
broader community of users. It does not provide a formal, complete
definition of quality, rather, it sets out a consistent means by
which information can be provided such that a potential user of a
dataset can make his/her own judgment about its fitness for purpose.
This document presents the most mature version of the Data Quality Vocabulary that could be produced in the lifespan of the Data on the Web Best Practices Working Group. At time of publication, its main components have remained stable for several months, even after receiving feedback and suggestions from the community. We expect however that further clarifications and extensions of this model may be carried out by future working groups, considering requirements from specific domains or applications.
Introduction
The Data on the Web Best Practices have pointed out
the relevance of publishing
information about the quality of data published on the Web.
Accordingly, the W3C Data on the Web Best Practices Working Group has been chartered to create
a vocabulary for expressing data quality. The Data Quality Vocabulary
(DQV) presented in this document is foreseen as an extension to the DCAT
vocabulary [[vocab-dcat]] to cover the quality of the data, how frequently is it
updated, whether it accepts user corrections, persistence commitments
etc. When used by publishers, this vocabulary will foster trust in
the data amongst developers.
This vocabulary does not seek to determine what
"quality" means. We believe that quality lies in the eye of
the beholder; that there is no objective, ideal definition of it.
Some datasets will be judged as low-quality resources by some data
consumers, while they will perfectly fit others' needs. In
accordance, we attach a lot of importance to allowing many actors to
assess the quality of datasets and publish their annotations,
certificates, opinions about a dataset. A dataset's publisher should
seek to publish metadata that helps data consumers determine whether
they can use the dataset to their benefit. However, publishers should
not be the only ones to have a say on the quality of data published
in an open environment like the Web. Certification agencies, data
aggregators, data consumers can make relevant quality assessments,
too.
We want to stimulate this by making it easier to publish,
exchange and consume quality metadata, for every step of a dataset's
lifecycle. This is why next to rather expected constructs like
quality measurements, the Data Quality Vocabulary puts an emphasis
on feedback, annotation, policies and certificates.
DQV draws inspiration from and is aligned with existing work on representing data quality, notably
the daQ ontology for representing information (especially metrics) on the quality
of linked open datasets [[DaQ]].
daQ is itself anchored in the RDF Data Cube framework
for publishing statistical data
[[Vocab-Data-Cube]].
Note that DQV elements can be applied not only to
express metadata on the quality of datasets; they can also be used to express
statements about the quality of that metadata itself. This is especially true when it
comes to representing the provenance of that metadata or its conformance with respect
to established metadata standards.
A list of DQV-related implementations including references to vocabulary extending DQV, tools serializing quality results in DQV, portals exposing data quality in DQV, and papers citing the DQV, is maintained at the DQV Implementation wiki page. Please feel free to contact the editors if you have any implementation to report.
Namespaces
The namespace for DQV is http://www.w3.org/ns/dqv#.
DQV, however, seeks to re-use elements from other vocabularies,
notably DCAT, following the
best
practices for data vocabularies identified by the Data on the Web
Best Practices Working Group.
The table below indicates the full list of namespaces and
prefixes used in this document.
Prefix
Namespace
daq
http://purl.org/eis/vocab/daq#
dcat
http://www.w3.org/ns/dcat#
dcterms
http://purl.org/dc/terms/
dqv
http://www.w3.org/ns/dqv#
duv
http://www.w3.org/ns/duv#
oa
http://www.w3.org/ns/oa#
prov
http://www.w3.org/ns/prov#
sdmx-attribute
http://purl.org/linked-data/sdmx/2009/attribute#
skos
http://www.w3.org/2004/02/skos/core#
Vocabulary Overview
The following vocabulary is based on DCAT [[vocab-dcat]] that
it extends with a number of additional properties and classes
suitable for expressing the quality of a dataset.
The quality of a given dataset or distribution is assessed via a
number of observed properties. For instance, one may consider a
dataset to be of high quality because it conforms to a specific
standard while for other use-cases the quality of the data will
depend on its level of interlinking with other datasets. To express
these properties an instance of a dcat:Dataset
or dcat:Distribution
can be related to five different types of quality information represented by the following classes:
dqv:QualityAnnotation
represents feedback and quality certificates given about the
dataset or its distribution.
dcterms:Standard
represents a standard the dataset or its distribution conforms to.
dqv:QualityPolicy
represents a policy or agreement that is chiefly governed by data quality concerns.
dqv:QualityMeasurement
represents a metric value providing quantitative or qualitative
information about the dataset or distribution.
prov:Entity
represents an entity involved in the provenance of the dataset or
distribution.
DQV defines quality measures as specific instances of Quality Measurements, adapting the daQ quality framework [[DaQ]], [[DaQ-RDFCUBE]]. It relies on quality dimensions and quality metrics.
A Quality Dimension (dqv:Dimension) is a quality-related characteristic of a dataset
relevant to the consumer (e.g., the availability of a dataset).
A Quality Metric (dqv:Metric) gives a procedure for measuring a data quality dimension, which is abstract, by observing a concrete quality indicator. There are usually multiple metrics per dimension; e.g., availability can be indicated by the accessibility of a SPARQL endpoint, or that of an RDF dump. The value of a metric can be numeric (e.g., for the metric “human-readable labeling of classes, properties and entities”, the percentage of entities having an rdfs:label or rdfs:comment) or boolean (e.g., whether or not a SPARQL endpoint is accessible).
Besides quality measurements, DQV considers certificates, standards, and quality policies, which can also be organized according to dimensions. Quality metadata containers (dqv:QualityMetadata) can group together different quality statements, so that their provenance can be tracked jointly.
Data model showing the main relevant classes
and their relations.
N.B.: "containment" refers to the inclusion of quality statements into "containers", which may or may not be treated as (RDF) graphs (see later example and the usage note for the class dqv:QualityMetadata).
Quality information can be derived from other quality information. For example, a quality annotation can be derived from a standard or a quality measurement. Quality measurements can be derived from other measurements. Metrics can be derived from other metrics. A standard can be built on another standard or a (set of) metrics. DQV models such derivations through the property prov:wasDerivedFrom as illustrated in the diagram below.
Using the property prov:wasDerivedFrom to interrelate quality metrics and other quality statements.
Subclassing between DQV and W3C's Provenance Ontology [[PROV-O]] has not been explicitly stated in this document, as PROV-O classes can be inferred by the use of PROV-O relations. This editorial choice is not meant to limit in any way the use of PROV-O with DQV. In fact, specific examples of PROV-O exploitation have been included to express the derivation between quality metrics, measurements and annotations, and to document the provenance of quality metadata and single quality measurements.
Vocabulary specification
Some properties from third-party vocabularies like Dublin Core [[Dublin-Core]], RDF Data Cube [[Vocab-Data-Cube]] and Web Annotation [[WebAnnotation]] are included here in order to make the document more self-contained and better readable. The definitions of these properties are copied from their original specifications and the way these properties should be used in the context of Data Quality Vocabulary is explained in the DQV usage notes.
The unit of measure in quality measurement should be specified through the property sdmx-attribute:unitMeasure as recommended by RDF Data Cube [[Vocab-Data-Cube]].
The Ontology of units of Measure (OM) [[RijgersbergEtAl]] provides a list of HTTP dereferenceable unit of measures, which can be exploited as values for sdmx-attribute:unitMeasure.
Indicates the dataset to which a quality measurement (which is an
RDF Data Cube observation) belongs. This is not the dataset whose quality
is being measured, which is indicated by dqv:computedOn.
Refers to the resource (e.g., a dataset, a linkset, a
graph, a set of triples) on which the quality measurement is
performed. In the DQV context, this property is generally
expected to be used in statements in which objects are instances
of dcat:Dataset
or dcat:Distribution.
Represents criteria relevant for assessing quality. Each
quality dimension must have one or more metric to measure it.
A dimension is linked with a category using the dqv:inCategory
property.
Categories are meant to systematically organize
dimensions. The Data Quality Vocabulary defines no specific
cardinality constraints for dqv:inCategory, since distinct
quality frameworks might have different perspectives over a
dimension. A dimension may therefore be associated to more than
one category. However, those who define new quality metrics
should try to avoid this as much as possible and assign only one
category to the dimensions they define.
Dimension and category are abstract entities. We represent instances dqv:Dimension and dqv:Category as instances of skos:Concept, which we think enable similar features as these for dimensions and categories in daQ. Our representation choice differs significantly for metrics, however. daQ uses RDFS/OWL classes and subclasses so as to represent constraints on measurements (e.g., on the type of values). RDFS/OWL does not allow one to fully capture all constraints. Additionally, other languages are being defined to represent constraints in more appropriate ways. We think it is therefore not appropriate now to recommend to treat specific metrics as subclasses of dqv:Metric, and we refer implementers to future progress on SHACL and related technology. See Appendix "Defining and using parameters for metrics" for further discussion.
In DQV, each dataset of quality measurements is an RDF Cube DataSet. This property can be used to indicate the RDF Data structure definition a dataset of quality measurements is expected to comply with.
This property is used to attach an instance of dqv:QualityPolicy to a dataset, a linkset, a graph, a set of triples, etc. In the DQV context, this property is generally expected to be used in statements in which objects are instances of dcat:Dataset or dcat:Distribution.
Represents quality annotations, including ratings, quality
certificates or feedback that can be associated to datasets or
distributions. Quality annotations must have one oa:motivatedBy
statement with an instance of oa:Motivation (and skos:Concept) that reflects a quality assessment purpose. We define this
instance as dqv:qualityAssessment.
This property is used in DQV to connect an instance of dqv:QualityAnnotation or its subclasses (dqv:QualityCertificate and dqv:UserQualityFeedback) to the resource the annotation is about. Any kind of resource (e.g., a dataset, a linkset, a graph, a set of triples) could be a target. However, in the
DQV context, this property is generally expected to be used in
statements in which objects are instances of dcat:Dataset
or dcat:Distribution.
The Web Annotation Vocabulary [[WebAnnotation]] is intended as a W3C Recommendation, but it is still under development at the time of publishing this Note. We have made our best to consider the latest version available, but changes might take place until it becomes a formal Recommendation. For example, new kinds of annotation body might be included and supersede the types we have considered in DQV. For this reason, we invite readers to check examples in this Note with the latest version of the Web Annotation Vocabulary.
An annotation that associates a resource (especially, a dataset or a distribution) to another resource (for example, a document) that certifies the resource's quality according to a set of quality assessment rules.
Future DQV versions may opt for renaming this class. The current name is a little misleading, as it hints that instances of this class are quality certificates rather than annotations pointing to quality certificates. Feedback is welcome!
Represents feedback that users have on the quality of datasets or distributions. Besides dqv:qualityAssessment, which is the motivation required by all quality annotations, one of the predefined instances of oa:Motivation should be indicated as motivation to distinguish among the different kinds of feedback, e.g., classifications, questions.
QualityMetadata containers do not necessary include all types of quality statements DQV can support. Implementers decide the granularity of containment. In the current version of DQV, we also leave open the choice of the containment "technique". Implementers can use (RDF) graph containment. They may also use an appropriate property of their choice — for example (a subproperty of) dcterms:hasPart — to link instances of dqv:QualityMetadata with instances of other DQV classes.
Generation is the completion of production of a new entity by an activity. This entity did not exist before generation and becomes available for usage after this generation.
Dimensions are meant to systematically organize metrics, quality certificates and quality annotations.
The Data Quality Vocabulary defines no specific cardinality
constraints for dqv:inDimension, since distinct quality
frameworks might have different perspectives over a metric. A
metric may therefore be associated to more than one dimension.
However, those who define new quality metrics should try to
avoid this as much as possible and assign only one dimension to
the metrics they define. More than one dimension can be indicated for each quality annotation or certificate.
Refers to the performed quality measurements. Quality
measurements can be performed to any kind of resource (e.g., a
dataset, a linkset, a graph, a set of triples). However, in the
DQV context, this property is generally expected to be used in
statements in which subjects are instances of dcat:Dataset
or dcat:Distribution.
Refers to a grouping of quality information such as certificates, policies, measurements and annotations as a named graph. Quality information represented in such a grouping can pertain to any kind of resource (e.g., a
dataset, a linkset, a graph, a set of triples). However, in the
DQV context, this property is generally expected to be used in
statements in which subjects are instances of dcat:Dataset
or dcat:Distribution.
Refers to a quality annotation. Quality
annotation can be applied to any kind of resource, e.g., a
dataset, a linkset, a graph, a set of triples. However, in the
DQV context, this property is generally expected to be used in
statements in which subjects are instances of dcat:Dataset
or dcat:Distribution.
A derivation is a transformation of an entity into another, an update of an entity resulting in a new one, or the construction of a new entity based on a pre-existing entity.
prov:wasDerivedFrom expresses a quite abstract relation of derivation. More specialized relations of derivation can be defined as subproperties of prov:wasDerivedFrom, whenever this is required by applications.
Whenever DQV implementers need to extend the motivations for quality annotations, they should follow the instructions provided by the Web Annotation Data Model, and the concepts in the extension should be defined as specializations of dqv:qualityAssessment.
Precision is a quality dimension, which refers to the recorded level of details. It represents the exactness of a measurement or description. It is equivalent the notion of Precision from ISO 25012.
The above section presents the most important classes and properties that DQV uses
to represent quality-related metadata. I.e., these are the elements needed to express
the machine-readable information that DQV-aware data consumers can exploit
for their processes. However, in many cases extra human-readable information is also needed,
e.g., to provide (multilingual) definitions and labels or just more context for the quality
measurements, metrics, etc. that are expressed with the core DQV classes.
To this end, we recommend using appropriate vocabularies like SKOS [[SKOS-reference]], Dublin Core [[Dublin-Core]] or PROV [[PROV-O]].
SKOS and PROV provide elements that fit specific purposes but can be applied to a wide
range of resources, i.e., they minimize their ontological commitment.
The section "Express a quality assessment with quality metrics"
shows examples with the properties skos:prefLabel and
skos:definition. These properties can be applied to instances of
dqv:Dimension, which are by default instances of
skos:Concept, but also to instances of
dqv:Metric, which are not. Dublin Core provides general-purpose
metadata elements that can also be used to express, say, a general description of a measurement:
:currentnessMeasurement a dqv:QualityMeasurement ;
dqv:value "false"^^xsd:boolean
dcterms:description “The triples in the dataset are outdated” .
Example Usage
This section shows some examples to illustrate the application of the
Dataset Quality Vocabulary.
NB: in the remainder of this section, the prefix ":"
refers to http://example.org/
Express a quality assessment with quality metrics
Let us consider a dataset
:myDataset
, and its distribution
:myDatasetDistribution
,
:myDataset
a dcat:Dataset ;
dcterms:title "My dataset" ;
dcat:distribution :myDatasetDistribution
.
:myDatasetDistribution
a dcat:Distribution ;
dcat:downloadURL <http://www.example.org/files/mydataset.csv> ;
dcterms:title "CSV distribution of dataset" ;
dcat:mediaType "text/csv" ;
dcat:byteSize "87120"^^xsd:decimal
.
An automated quality checker has provided a quality assessment with
two (CSV) quality measurements for
:myDatasetDistribution
.
:myDatasetDistribution
dqv:hasQualityMeasurement :measurement1, :measurement2
.
:measurement1
a dqv:QualityMeasurement ;
dqv:computedOn :myDatasetDistribution ;
dqv:isMeasurementOf :downloadURLAvailabilityMetric ;
dqv:value "true"^^xsd:boolean
.
:measurement2
a dqv:QualityMeasurement ;
dqv:computedOn :myDatasetDistribution ;
dqv:isMeasurementOf :csvCompletenessMetric ;
dqv:value "0.5"^^xsd:double
.
#definition of dimensions and metrics
:availability
a dqv:Dimension ;
skos:prefLabel "Availability"@en ;
skos:definition "Availability of a dataset is the extent to which data (or some
portion of it) is present, obtainable and ready for use."@en ;
dqv:inCategory :accessibility
.
:completeness
a dqv:Dimension ;
skos:prefLabel "Completeness"@en ;
skos:definition "Completeness refers to the degree to which all required information
is present in a particular dataset."@en ;
dqv:inCategory :intrinsicDimensions
.
:downloadURLAvailabilityMetric
a dqv:Metric ;
skos:definition "It checks if dcat:downloadURL is available and if its value is
dereferenceable."@en ;
dqv:expectedDataType xsd:boolean ;
dqv:inDimension :availability
.
:csvCompletenessMetric
a dqv:Metric ;
skos:definition "Ratio between the number of objects represented in the csv and the
number of objects expected to be represented according to the declared dataset
scope."@en ;
dqv:expectedDataType xsd:double ;
dqv:inDimension :completeness
.
Categories and dimensions might be more extensively defined,
see in the section 'Dimensions and metrics hints' for further
examples. Any quality framework is free to define its own dimensions
and categories.
Document the provenance of the quality metadata
The results of metrics obtained in the previous assessment are
stored in the
:myQualityMetadata
graph.
# :myQualityMatadata is a graph
:myQualityMetadata {
:myDatasetDistribution
dqv:hasQualityMeasurement :measurement1, :measurement2
.
# The graph contains the rest of the statements presented in the previous example.
}
# :myQualityMetadata has been created by :myQualityChecker and it is the result of the
# :myQualityChecking activity
:myQualityMetadata
a dqv:QualityMetadata ;
prov:wasAttributedTo :myQualityChecker ;
prov:generatedAtTime "2015-05-27T02:52:02Z"^^xsd:dateTime ;
prov:wasGeneratedBy :myQualityChecking .
# :myQualityChecker is a service computing some quality metrics
:myQualityChecker
a prov:SoftwareAgent ;
rdfs:label "A quality assessment service"^^xsd:string .
# Further details about quality service/software can be provided, for example,
# deploying vocabularies such as Dataset Usage Vocabulary (DUV), Dublin Core or ADMS.SW
# :myQualityChecking is the activity that has generated :myQualityMetadata from
# :myDatasetDistribution
:myQualityChecking
a prov:Activity;
rdfs:label "The checking of myDatasetDistribution's quality"^^xsd:string;
prov:wasAssociatedWith :myQualityChecker;
prov:used :myDatasetDistribution;
prov:generated :myQualityMetadata;
prov:endedAtTime "2015-05-27T02:52:02Z"^^xsd:dateTime;
prov:startedAtTime "2015-05-27T00:52:02Z"^^xsd:dateTime
.
Document the provenance of single quality measurements
The group has discussed provenance at different levels of
granularity (dqv:QualityMeasurement and dqv:QualityMetadata). In the previous example we have shown how to track provenance at the level of quality metadata as a whole. In the following, we provide an example of provenance for a single quality measurement, :myMeasurement.
:myDatasetDistribution
dqv:hasQualityMeasurement :myMeasurement .
# :myMeasurement has been created by :myQualityChecker and it is the result of the
# :myQualityChecking activity
:myMeasurement
a dqv:QualityMeasurement ;
dqv:computedOn :myDatasetDistribution ;
dqv:isMeasurementOf :downloadURLAvailabilityMetric ;
dqv:value "true"^^xsd:boolean ;
prov:wasAttributedTo :myQualityChecker ;
prov:generatedAtTime "2015-05-27T02:52:02Z"^^xsd:dateTime ;
prov:wasGeneratedBy :myQualityChecking
.
:downloadURLAvailabilityMetric
a dqv:Metric ;
skos:definition "Checks if dcat:downloadURL is available and if its value is
dereferenceable."@en ;
dqv:expectedDataType xsd:boolean ;
dqv:inDimension :availability .
# :myQualityChecker is a service computing some quality metrics
:myQualityChecker
a prov:SoftwareAgent ;
rdfs:label "A quality assessment service"^^xsd:string .
# Further details about quality service/software can be provided, for example,
# deploying vocabularies such as Dataset Usage Vocabulary (DUV), Dublin Core or ADMS.SW
# :myQualityChecking is the activity that has generated :myMeasurement from :myDatasetDistribution
:myQualityChecking
a prov:Activity;
rdfs:label "The checking of myDatasetDistribution's quality"^^xsd:string;
prov:wasAssociatedWith :myQualityChecker;
prov:used :myDatasetDistribution;
prov:generated :myMeasurement;
prov:endedAtTime "2015-05-27T02:52:02Z"^^xsd:dateTime;
prov:startedAtTime "2015-05-27T00:52:02Z"^^xsd:dateTime .
Document the provenance of a dataset
Statements similar to the ones applied to the resource
:myQualityMetadata
above can be applied to the resource
:myDataset
to indicate the provenance of the dataset. I.e., a dataset can be
generated by a specific software agent, be generated at a certain
time, etc. The HCLS
Community Profile for describing datasets provides further
examples.
Express that a dataset received an ODI certificate
Let us express that an ODI certificate for the "City of
Raleigh Open Government Data" dataset is available at the URL
<https://certificates.theodi.org/en/datasets/393/certificate>.
<https://certificates.theodi.org/en/datasets/393> a dcat:Dataset ;
dqv:hasQualityAnnotation :myDatasetQA .
:myDatasetQA
a dqv:QualityCertificate ;
oa:hasTarget <https://certificates.theodi.org/en/datasets/393> ;
oa:hasBody <https://certificates.theodi.org/en/datasets/393/certificate> ;
oa:motivatedBy dqv:qualityAssessment
.
Express a question about dataset quality
Let us ask a question about the completeness of the "City of
Raleigh Open Government Data" dataset.
<https://certificates.theodi.org/en/datasets/393> a dcat:Dataset ;
dqv:hasQualityAnnotation :questionQA .
:questionQA
a dqv:UserQualityFeedback ;
oa:hasTarget <https://certificates.theodi.org/en/datasets/393> ;
oa:hasBody :textBody ;
oa:motivatedBy dqv:qualityAssessment, oa:questioning ;
dqv:inDimension :completeness
.
:textBody a oa:TextualBody ;
rdf:value "Could you please provide information about the completeness of your
dataset?" ;
dc:language "en" ;
dc:format "text/plain"
.
Express that a dataset fits in a quality classification
Let us express that the "City of
Raleigh Open Government Data" dataset is classified as a four stars
dataset against the 5 Stars Linked Open Data rating system.
<https://certificates.theodi.org/en/datasets/393> a dcat:Dataset ;
dqv:hasQualityAnnotation :classificationQA .
:classificationQA
a dqv:UserQualityFeedback ;
oa:hasTarget <https://certificates.theodi.org/en/datasets/393> ;
oa:hasBody :four_stars ;
oa:motivatedBy dqv:qualityAssessment, oa:classifying ;
dqv:inDimension :availability .
:four_stars
a skos:Concept;
skos:inScheme :OpenData5Star ;
skos:prefLabel "Four stars"@en ;
skos:definition "Dataset available on the Web with structured machine-readable non
proprietary format. It uses URIs to denote things."@en .
Express derivation between quality metrics, measurements and annotations
DQV models derivation with the property prov:wasDerivedFrom.
For example, the accessibility of the dataset :myDataset can be derived from the accessibility of its distributions :myCSVDatasetDistribution and :mySPARQLDatasetDistribution.
:myDataset
a dcat:Dataset ;
dcterms:title "My dataset" ;
dcat:distribution :myCSVDatasetDistribution, :mySPARQLDatasetDistribution
.
:myCSVDatasetDistribution
a dcat:Distribution ;
dcat:downloadURL <http://www.example.org/files/mydataset.csv> ;
dcterms:title "CSV distribution of dataset" ;
dcat:mediaType "text/csv" ;
dcat:byteSize "87120"^^xsd:decimal
.
:mySPARQLDatasetDistribution
a dcat:Distribution ;
dcat:accessURL <http://www.example.org/sparql>
dcterms:title "SPARQL access to the dataset" ;
dcat:mediaType "application/sparql-results+json"
.
# definition of dimensions and metrics
:availability
a dqv:Dimension ;
skos:prefLabel "Availability"@en ;
skos:definition "Availability of a dataset is the extent to which data (or some
portion of it) is present, obtainable and ready for use."@en ;
dqv:inCategory :accessibility
.
:downloadURLAvailabilityMetric
a dqv:Metric ;
skos:definition "Checks if dcat:downloadURL is available and if its value
is dereferenceable."@en ;
dqv:expectedDataType xsd:boolean ;
dqv:inDimension :availability
.
:SPARQLAvailabilityMetric
a dqv:Metric ;
skos:definition "Checks if an URL specified in dcat:accessURL is available
and if at that URL a SPARQL endpoint is active."@en ;
dqv:expectedDataType xsd:boolean ;
dqv:inDimension :availability
.
:datasetAvailabilityMetric
a dqv:Metric ;
prov:wasDerivedFrom :downloadURLAvailabilityMetric, :SPARQLAvailabilityMetric;
skos:definition "Checks the availabitity of the specified distributions."@en ;
dqv:expectedDataType xsd:boolean ;
dqv:inDimension :availability
.
Depending on the specific application context, the expression of this derivation can be kept at level of the quality measurements. In the following, the measurement :measurement3 of :myDataset's availability is derived from :measurement1 and :measurement2.
The classification of :myDataset as :three_stars can be derived from the result of a quality measurement :measurement2
:myDataset
dqv:hasQualityAnnotation :myDatasetClassification .
:myDatasetClassification
a dqv:UserQualityFeedback ;
prov:wasDerivedFrom :measurement2 ;
oa:hasTarget :myDataset ;
oa:hasBody :three_stars ;
oa:motivatedBy dqv:qualityAssessment, oa:classifying ;
dqv:inDimension :availability
.
:three_stars
a skos:Concept;
skos:inScheme :OpenData5Star ;
skos:prefLabel "three stars"@en ;
skos:definition "Dataset available on the Web with structured machine-readable
non proprietary format."@en
.
Express quality of SKOS concept schemes
Let’s consider
:myControlledVocabulary, a controlled vocabulary made available on the Web using the SKOS
[[SKOS-reference]] and DCAT [[vocab-dcat]].
:myControlledVocabulary
a dcat:Dataset ;
dcterms:title "My controlled vocabulary"
.
:myControlledVocabularyDistribution
a dcat:Distribution ;
dcat:downloadURL <http://www.example.org/files/myControlledVocabulary.ttl> ;
dcterms:title "SKOS/RDF distribution of my controlled vocabulary" ;
dcat:mediaType "text/turtle" ;
dcat:byteSize "190120"^^xsd:decimal
.
qSKOS is an open source tool, which detects quality issues
affecting SKOS vocabularies [[qSKOS]]. It considers 26 quality
issues including, for example, “Incomplete Language Coverage” and
“Label Conflicts” which are grouped in the category “Labeling and
Documentation issues”. Quality issues addressed by qSKOS can be
considered as DQV quality dimensions, whilst the number of concepts
in which a quality issue occurs can be the metric deployed for each
quality dimension.
# definition of instances for some of the metrics, dimensions and categories deployed
# in qSKOS.
:numOfConceptsWithLabelConflicts
a dqv:Metric;
skos:prefLabel "Conflicting concepts"@en ;
skos:definition "Number of concepts having conflicting labels"@en ;
dqv:expectedDataType xsd:interger ;
dqv:inDimension :LabelConflicts
.
:numOfConceptsWithIncompleteLanguageCoverage
a dqv:Metric;
skos:prefLabel "Language incomplete concepts"@en ;
skos:definition "Number of concepts having an incomplete language coverage"@en ;
dqv:expectedDataType xsd:interger ;
dqv:inDimension :incompleteLanguageCoverage .
:LabelConflicts
a dqv:Dimension;
skos:prefLabel "Label Conflicts"@en ;
skos:definition "Dimension corresponding to the label conflicts quality issue"@en ;
dqv:inCategory :labelingDocumentationIssues .
:incompleteLanguageCoverage
a dqv:Dimension;
skos:prefLabel "Incomplete Language Coverage"@en ;
skos:definition "Dimension corresponding to the incomplete language coverage
issue"@en ;
dqv:inCategory :labelingDocumentationIssues .
:labelingDocumentationIssues
a dqv:Category ;
skos:prefLabel "Labeling and Documentation Issues"@en ;
skos:definition "Category grouping labeling and documentation issues"@en
.
DQV represents the qSKOS quality assessment on
:myControlledVocabulary
for the dimensions “Incomplete Language Coverage” and “Label
Conflicts”.
(VoID) linksets are
collections of (RDF) links between two datasets: the linkset's subject and the linkset's object, containing respectively the subjects and objects of all link statements. Linksets are as
important as datasets when it comes to the joint exploitation of
datasets served independently in Linked Data. The representation of
quality for a linkset is another example of how DQV can be
exploited.
Let’s define three DCAT datasets, including one VoID linkset,
which connects a subject dataset to an object dataset named respectively by void:subjectsTarget and by void:objectsTarget:
:myDataset1
a dcat:Dataset ;
dcterms:title "My dataset 1"
.
:myDataset2
a dcat:Dataset ;
dcterms:title "My dataset 2"
.
:myLinkset
a dcat:Dataset, void:Linkset ;
dcterms:title "A Linkset from My dataset 1 to My dataset 2";
void:linkPredicate skos:exactMatch ;
void:subjectsTarget :myDataset1 ;
void:objectsTarget :myDataset2
.
We can represent information about the quality of :myLinkset
using the “Multilingual importing”
quality metric [[MultilingualImporting]]. This metric applies to linksets between datasets that
include SKOS concepts [[SKOS-reference]]. It considers the concepts that have been matched and quantifies the
information gain the links provide if one adds the preferred labels or the alternative
labels of the concepts from the object dataset to the descriptions of
the concepts from the subject dataset. Let us
first define the proper metric, dimension and category.
# Definition of instances for Metric, Dimension and Category.
:importingForPropertyPercentage
a dqv:Metric ;
skos:definition "Ratio between novel preferred or alternative labels
gained from the object dataset via skos:exactMatch links and the preferred or
alternative labels that are in the subject and object datasets."@en
dqv:expectedDataType xsd:double ;
dqv:inDimension :completeness .
:completenessGain
a dqv:Dimension ;
skos:prefLabel "Completeness Gain"@en ;
skos:definition "Degree to which a linkset
contribute to add relevant information to a particular dataset."@en ;
dqv:inCategory :complementationGain
.
:complementationGain
a dqv:Category ;
skos:definition "Category that groups dimensions measuring the data quality gain
obtained by exploiting linksets."@en
.
The quality assessment of the "label importing" depends on
two extra parameters: :onProperty and :onLanguage, which are
respectively, the SKOS property and the language tag considered for measuring the completeness gains. We add the following statements to represent these parameters (note that type qb:DimensionProperty and the range definitions are firstly needed for compatibility with RDF Data Cube).
:onLanguage
a qb:DimensionProperty, owl:DataProperty ;
rdfs:comment "Language on which label importing is assessed."@en ;
rdfs:domain dqv:QualityMeasurement;
rdfs:label "Label import assessment language"@en ;
rdfs:range xsd:string
.
:onProperty
a qb:DimensionProperty, rdf:Property ;
rdfs:comment "Property on which label importing is assessed."@en ;
rdfs:domain dqv:QualityMeasurement ;
rdfs:label "Label import assessment property"@en ;
rdfs:range rdf:Property
.
Let us add actual quality assessments for alternative and preferred labels in Italian and English:
## for Italian alternative labels
:measurement_exactMatchAltLabelIt
a dqv:QualityMeasurement;
dqv:computedOn :myLinkset ;
dqv:value "1.0"^^xsd:double ;
dcterms:date "2016-01-10"^^xsd:date ;
dqv:isMeasurementOf :importingForPropertyPercentage ;
:onLanguage "it" ;
:onProperty skos:altLabel .
## for English alternative labels
:measurement_exactMatchAltLabelEn
a dqv:QualityMeasurement;
dqv:computedOn :myLinkset ;
dqv:value "0.1"^^xsd:double ;
dcterms:date "2016-01-10"^^xsd:date;
dqv:isMeasurementOf :importingForPropertyPercentage ;
:onLanguage "en" ;
:onProperty skos:altLabel .
## for Italian preferred labels
:measurement_exactMatchPrefLabelIt
a dqv:QualityMeasurement;
dqv:computedOn :myLinkset ;
dqv:value "0.5"^^xsd:double ;
dcterms:date "2016-01-10"^^xsd:date ;
dqv:isMeasurementOf :importingForPropertyPercentage ;
:onLanguage "it" ;
:onProperty skos:prefLabel .
## for English preferred labels
:measurement_exactMatchPrefLabelEn
a dqv:QualityMeasurement;
dqv:computedOn :myLinkset ;
dqv:value "1"^^xsd:double ;
dcterms:date "2016-01-10"^^xsd:date ;
dqv:isMeasurementOf :importingForPropertyPercentage ;
:onLanguage "en" ;
:onProperty skos:prefLabel .
Express the conformance of a dataset's metadata with a standard
It is often desirable to indicate that metadata about datasets in a
catalogue are compliant with a metadata standard, or an application
profile of an existing metadata standard. A typical example is the
GeoDCAT Application Profile [[GeoDCAT-AP]], an extension of the DCAT
vocabulary [[vocab-dcat]] to represent metadata for geospatial data
portals. GeoDCAT-AP allows one to express that a dataset's metadata
conforms to an existing standard, following the recommendations of
ISO 19115, ISO 19157 and the EU INSPIRE directive. DCAT partly
supports the expression of such metadata conformance statements. The
following example illustrates how a (DCAT)
catalog record can be said to be conformant with the GeoDCAT-AP
standard itself.
:myDataset a dcat:Dataset .
:myDatasetRecord a dcat:CatalogRecord ;
foaf:primaryTopic :myDataset ;
dcterms:conformsTo :geoDCAT-AP .
:geoDCAT-AP a dcterms:Standard;
dcterms:title "GeoDCAT Application Profile. Version 1.0" ;
dcterms:comment "GeoDCAT-AP is developed in the context of the Interoperability
Solutions for European Public Administrations (ISA) Programme"@en;
dcterms:issued "2015-12-23"^^xsd:date ;
foaf:page
<https://joinup.ec.europa.eu/asset/dcat_application_profile/asset_release/geodcat-ap-v10>
.
Note that this example does not include the metadata about the
dataset :myDataset itself. We assume this is present in an RDF
data source accessible via the URI
:myDatasetRecord. We also assume that
:geoDCAT-AP
is a reference URI that denotes the GeoDCAT-AP standard, which can
be re-used across many catalog record descriptions, not just a
locally introduced URI.
Finer-grained representation of conformance statements can be found in the literature, and applications with more complex requirements may implement them, including for example the requirement of representing 'non-conformance' tested by specific procedures. The GeoDCAT Application Profile, for example, suggests a "provisional mapping" for extended profiles, which re-uses the PROV data model for provenance (see Annex II.14 at [[GeoDCAT-AP]]). Such patterns come however at the cost of having to publish and exchange representations that are much more elaborate. They will also have to be aligned with the result of another ongoing efforts on data validation and the reporting thereof, as currently discussed around SHACL, for example. We have thus decided to postpone addressing these requirements for now.
Express the conformance of a dataset with a quality policy
DQV introduces the class dqv:QualityPolicy to express that a Dataset or Distribution
follows a policy or agreement that is chiefly defined by data quality concerns.
DQV does not provide a complete framework for expressing policies.
The class dqv:QualityPolicy is rather meant as an anchor point, through which
DQV implementers can relate properties and classes of policy-dedicated vocabularies,
such as ODRL [[ODRL]], to the core
elements that define quality of datasets and distributions.
The example below specifies that a data provider grants the permission to access
a dataset and commits to serve the data with a certain quality, more concretely,
99% availability of a SPARQL endpoint (distribution) associated with the dataset.
This is expressed in ODRL as an offer with a duty on the service provider
that states a constraint defined using a DQV metric (:sparqlEndpointUptime), for which measurements
have to be greater than a certain percentage (99). The odrl:assigner is the issuer
of the policy statement; it is also the assignee of the duty to deliver the
distribution as the policy requires it. There is no explicitly mentioned recipient
for the policy itself, since this examples is about a generic data access scenario.
Note that instances of dqv:QualityPolicy could be instances of
the class odrl:Agreement,
in which case an odrl:assignee is likely to appear for the policy.
:serviceProvider a odrl:Party .
:myDataset a dcat:Dataset ;
dcat:distribution :myDatasetSparqlDistribution ;
:myDatasetSparqlDistribution a dcat:Distribution .
:policy1 a odrl:Offer, dqv:QualityPolicy ;
odrl:permission [
a odrl:Permission ;
odrl:target :myDataset ;
odrl:action odrl:read ;
odrl:assigner :serviceProvider;
odrl:duty [
a odrl:Duty;
odrl:assignee :serviceProvider;
odrl:target :myDatasetSparqlDistribution ;
odrl:constraint [
a odrl:Constraint ;
prov:wasDerivedFrom :sparqlEndpointUptime;
odrl:percentage "99"^^xsd:double ;
odrl:operator odrl:gteq
]
]
]
.
The expression of constraints in ODRL seems quite unfit with expressing
general constraints on values in RDF graphs, as we would require here.
However, ODRL can be easily extended, and is schedule to undergo refinement
in the context of the W3C Permissions & Obligations
Expression Working Group. In the future implementers should investigate whether a
general constraint expression language like the coming SHACL [[SHACL]]
provides a more appropriate mechanism to be
used on top of ODRL permissions and duties.
Express dataset precision and accuracy
The need for documenting data precision (also sometimes referred to as "resolution") is a common requirement, in particular, when dealing with spatial data. The following example shows how DQV can meet this requirement.
:myDataset a dcat:Dataset ;
dqv:hasQualityMeasurement :myDatasetPrecision, :myDatasetAccuracy .
:myDatasetPrecision a dqv:QualityMeasurement ;
dqv:isMeasurementOf :spatialResolutionAsDistance ;
dqv:value "1000"^^xsd:decimal ;
sdmx-attribute:unitMeasure <http://www.wurvoc.org/vocabularies/om-1.8/metre>
.
:spatialResolutionAsDistance a dqv:Metric;
skos:definition "Spatial resolution of a dataset expressed as distance"@en ;
dqv:expectedDataType xsd:decimal ;
dqv:inDimension dqv:precision
.
Precision can be alternatively expressed without unit of measure specifying spatial resolution by means of an "equivalent scale" with a fraction (e.g., 1:1,000, 1:1,000,000)
:myDataset a dcat:Dataset;
dqv:hasQualityMeasurement :myDatasetPrecisionES .
:myDatasetPrecisionES a dqv:QualityMeasurement ;
dqv:isMeasurementOf :spatialResolutionAsEquivalentScale ;
dqv:value "0.000001"^^xsd:decimal
.
:spatialResolutionAsEquivalentScale a dqv:Metric;
skos:definition "Spatial resolution of a dataset expressed as equivalent scale,
by using a representative fraction (e.g., 1:1,000, 1:1,000,000)."@en ;
dqv:expectedDataType xsd:decimal ;
dqv:inDimension dqv:precision
.
or specifying the angular distance.
:myDataset a dcat:Dataset;
dqv:hasQualityMeasurement :myDatasetPrecisionAS .
:myDatasetPrecisionAS a dqv:QualityMeasurement ;
dqv:isMeasurementOf :spatialResolutionAsAngularDistance ;
dqv:value "3.5"^^xsd:decimal ;
sdmx-attribute:unitMeasure <http://www.wurvoc.org/vocabularies/om-1.8/degree> .
:spatialResolutionAsAngularDistance a dqv:Metric;
skos:definition "Spatial resolution of a dataset expressed as angular distance"@en ;
dqv:expectedDataType xsd:decimal ;
dqv:inDimension dqv:precision
.
Note that the precision (or resolution) of a dataset is not equivalent to its accuracy. High precision values are not necessarily accurate. High precision values can even be pointless, as when one asserts that Magna Carta was signed at 1215-06-15T00:00:00. Accuracy is nonetheless an important dimension of data quality. Data accuracy metrics and measurements can be represented with DQV, as in the following example:
:myDatasetAccuracy a dqv:QualityMeasurement ;
dqv:isMeasurementOf :spatialAccuracy ;
dqv:value "98.2"^^xsd:decimal
sdmx-attribute:unitMeasure <http://www.wurvoc.org/vocabularies/om-1.8/Percentage>
.
:spatialAccuracy a dqv:Metric;
skos:definition "Percentage of spatial elements that are found accurate
according to methodology XYZ"@en ;
dqv:expectedDataType xsd:decimal ;
dqv:inDimension ldqd:semanticAccuracy
.
Dimensions and metrics hints
This section gathers relevant quality dimensions and ideas for
corresponding metrics, which might be eventually represented as
instances of dqv:Category, dqv:Dimension
and dqv:Metric.
The goal of the Data Quality Vocabulary is not to define a normative list of dimensions and metrics.
There are already several reference classifications available, which are the result of
a lot of community work. Unifying them here seems both hard and not desirable, as
fundamental approaches to quality vary between domains or even applications.
This section provides instead a set of examples, starting from use
cases included in the Use
Cases & Requirements document. In particular, we offer the quality dimension proposed in ISO 25012
[[ISOIEC25012]] and Zaveri et al. [[ZaveriEtAl]] as two starting points.
Ultimately, implementers will need to choose themselves the approach that fits best their needs.
They can extend on these starting points, creating their own refinements of categories and dimensions,
and of course their own metrics. They can mix existing approaches — we show that
the proposals from ISO and Zaveri et al. are not completely disjoint.
Implementers can also adopt completely different classifications, if existing ones do not
fit their specific application scenarios. They should however be aware that relying on existing classifications and metrics
increases interoperability, i.e., the chance that human and machine agents can properly understand and exploit their quality assessments.
Statistics
Statistics are not systematically conceived as quality measures.
Yet, statistics on datasets can be seen as quality indicators relevant for the dimensions listed in
the rest of this section. For instance, the percentages of empty string attributes and of URIs that
do not point to any useful information (either directly in the published data or via HTTP content negotiation)
can indicate quality issues for compliance or completeness. These indicators can be published as such
or aggregated with other statistics into further refined quality assessments, using the pattern
with prov:wasDerivedFrom
presented in the "Vocabulary Overview" section.
The following table gives examples of statistics that can be computed
on a dataset and interpreted as quality indicators for completeness and understandability, coming from the VoID
extension created for the Aether tool.
We invite readers interested in statistics to investigate whether elements coming from Aether or similar efforts
can match their needs.
Number of distinct external resources used (including
schema terms)
http://ldf.fi/void-ext#distinctIRIReferences
Number of distinct literals
http://ldf.fi/void-ext#distinctLiterals
Number of languages used
http://ldf.fi/void-ext#languages
The Aether VoID
extension represents statistics as direct statements that have a
dataset as subject and an integer as object. This pattern, which
can be expected to be rather common, is different from the pattern
that DQV inherits from daQ. Guidance on how DQV can work with
other quality statistics vocabulary shall be provided with future versions of the DQV documentation.
Quality dimensions defined in ISO/IEC 25012
ISO/IEC 25012
provides an example of quality dimensions
grouped in three categories that can be adopted to document the
quality of datasets. These quality dimensions and categories are
listed in the table below.
Category
Dimension
Definition
Inherent Data Quality
Accuracy
The degree to which data has attributes that correctly
represent the true value of the intended attribute of a concept or
event in a specific context of use.
Completeness
The degree to which subject data associated with an entity
has values for all expected attributes and related entity
instances in a specific context of use.
Consistency
The degree to which data has attributes that are free from
contradiction and are coherent with other data in a specific
context of use. It can be either or both among data regarding one
entity and across similar data for comparable entities.
Credibility
The degree to which data has attributes that are regarded
as true and believable by users in a specific context of use.
Credibility includes the concept of authenticity (the truthfulness
of origins, attributions, commitments).
Currentness
The degree to which data has attributes that are of the
right age in a specific context of use.
Inherent and System-Dependent Data Quality
Accessibility
The degree to which data can be accessed in a specific
context of use, particularly by people who need supporting
technology or special configuration because of some disability.
Compliance
The degree to which data has attributes that adhere to
standards, conventions or regulations in force and similar rules
relating to data quality in a specific context of use.
Confidentiality
The degree to which data has attributes that ensure that
it is only accessible and interpretable by authorized users in a
specific context of use. Confidentiality is an aspect of
information security (together with availability, integrity) as
defined in ISO/IEC 13335-1:2004.
Efficiency
The degree to which data has attributes that can be
processed and provide the expected levels of performance by using
the appropriate amounts and types of resources in a specific
context of use.
Precision
The degree to which data has attributes that are exact or
that provide discrimination in a specific context of use.
Traceability
The degree to which data has attributes that provide an
audit trail of access to the data and of any changes made to the
data in a specific context of use.
Understandability
The degree to which data has attributes that enable it to
be read and interpreted by users, and are expressed in appropriate
languages, symbols and units in a specific context of use. Some
information about data understandability are provided by metadata.
System-Dependent Data Quality
Availability
The degree to which data has attributes that enable it to
be retrieved by authorized users and/or applications in a specific
context of use.
Portability
The degree to which data has attributes that enable it to
be installed, replaced or moved from one system to another
preserving the existing quality in a specific context of use.
Recoverability
The degree to which data has attributes that enable it to
maintain and preserve a specified level of operations and quality,
even in the event of failure, in a specific context of use.
DQV can express the dimensions and categories listed in the
table above. The following example includes only an exemplification of the ISO dimensions and categories, which could be authoritatively provided by ISO. Semantic relation defined in SKOS can be exploited to related categories and dimensions. For example, in the following, skos:broader has been exploited to define iso:inherentSystemDependentDataQuality as a specialization of iso:inherentDataQuality and iso:systemDependentDataQuality.
# definition of ISO categories
iso:inherentDataQuality a dqv:Category ;
skos:prefLabel "Inherent Data Quality"@en.
iso:systemDependentDataQuality a dqv:Category ;
skos:prefLabel "System-Dependent Data Quality"@en.
iso:inherentSystemDependentDataQuality a dqv:Category ;
skos:prefLabel "Inherent and System-Dependent Data Quality"@en.
skos:broader iso:inherentDataQuality, iso:systemDependentDataQuality .
# definition of ISO dimensions
iso:accuracy a dqv:Dimension ;
dqv:inCategory iso:inherentDataQuality ;
skos:prefLabel "Accuracy"@en;
skos:definition "The degree to which data has attributes that correctly represent
the true value of the intended attribute of a concept or event in a specific context
of use."@en
.
iso:completeness a dqv:Dimension ;
dqv:inCategory iso:inherentDataQuality ;
skos:prefLabel "Completeness"@en;
skos:definition "The degree to which subject data associated with an entity has
values for all expected attributes and related entity instances in a specific context
of use."@en
.
iso:consistency a dqv:Dimension ;
dqv:inCategory iso:inherentDataQuality ;
skos:prefLabel "Consistency"@en;
skos:definition "The degree to which data has attributes that are free from
contradiction and are coherent with other data in a specific context of use.
It can be either or both among data regarding one entity and across similar
data for comparable entities."@en
.
# ... ...
iso:accessibility a dqv:Dimension ;
dqv:inCategory iso:inherentSystemDependentDataQuality ;
skos:prefLabel "Accessibility"@en;
skos:definition "The degree to which data can be accessed in a specific context of
use, particularly by people who need supporting technology or special configuration
because of some disability."@en
.
# ... etc ...
Quality dimensions defined for Linked Data
Zaveri et al. provides a review of quality dimensions,
which is specifically suited for Linked Data [[ZaveriEtAl]].
Category
Dimension
Definition
Accessibility dimensions
Availability
Availability of a dataset is the extent to which data (or
some portion of it) is present, obtainable and ready for use.
Licensing
Licensing is defined as the granting of permission for a
consumer to re-use a dataset under defined conditions.
Interlinking
Interlinking refers to the degree to which entities that
represent the same concept are linked to each other, be it within
or between two or more data sources.
Security
Security is the extent to which data is protected against
alteration and misuse.
Performance
Performance refers to the efficiency of a system that
binds to a large dataset, that is, the more performant a data
source is the more efficiently a system can process data.
Intrinsic dimensions
Syntactic validity
Syntactic validity is defined as the degree to which an
RDF document conforms to the specification of the serialization
format.
Semantic accuracy
Semantic accuracy is defined as the degree to which data
values correctly represent the real world facts.
Consistency
Consistency means that a knowledge base is free of
(logical/formal) contradictions with respect to particular
knowledge representation and inference mechanisms.
Conciseness
Conciseness refers to the minimization of redundancy of
entities at the schema and the data level.
Completeness
Completeness refers to the degree to which all required
information is present in a particular dataset.
Contextual dimensions
Relevancy
Relevancy refers to the provision of information which is
in accordance with the task at hand and important to the users’
query.
Trustworthiness
Trustworthiness is defined as the degree to which the
information is accepted to be correct, true, real and credible.
Understandability
Understandability refers to the ease with which data can
be comprehended without ambiguity and be used by a human
information consumer.
Timeliness
Timeliness measures how up-to-date data is relative to a
specific task.
Representational dimensions
Representational-conciseness
Representational-conciseness refers to the representation of the data, which is compact and well formatted.
Interoperability
Interoperability is the degree to which the format and
structure of the information conforms to previously returned
information as well as data from other sources.
Interpretability
Interpretability refers to technical aspects of the data,
that is, whether information is represented using an appropriate
notation and whether the machine is able to process the data.
Versatility
Versatility refers to the availability of the data in
different representations and in an internationalized way.
DQV can express these dimensions and categories as shown in the following example.
The encoding of all the dimensions and categories mentioned above can be found at https://www.w3.org/2016/05/ldqd.
# definition of categories from Zaveri et al
ldqd:accessibilityDimensions a dqv:Category ;
skos:prefLabel "Accessibility"@en.
ldqd:intrinsicDimensions a dqv:Category ;
skos:prefLabel "Intrinsic dimensions"@en.
ldqd:contextualDimensions a dqv:Category ;
skos:prefLabel "Contextual dimensions"@en.
ldqd:representationalDimensions a dqv:Category ;
skos:prefLabel "Representational Dimensions"@en.
#definition of dimensions from Zaveri et al
ldqd:availability
a dqv:Dimension ;
dqv:inCategory ldqd:accessibilityDimensions ;
skos:prefLabel "Availability"@en;
skos:definition "Availability of a dataset is the extent to which data (or some
portion of it) is present, obtainable and ready for use."@en
.
ldqd:licensing
a dqv:Dimension ;
dqv:inCategory ldqd:accessibilityDimensions ;
skos:prefLabel "Licensing"@en;
skos:definition "Licensing is defined as the granting of permission for a consumer to
re-use a dataset under defined conditions."@en
.
ldqd:interlinking
a dqv:Dimension ;
dqv:inCategory ldqd:accessibilityDimensions ;
skos:prefLabel "Consistency"@en;
skos:definition "Interlinking refers to the degree to which entities that represent
the same concept are linked to each other, be it within or between two or more data
sources."@en
.
# ... etc ...
Expressing relations between quality dimensions
In Zaveri Et Al. [[ZaveriEtAl]] some dimensions
are not completely independent and may be related. These relationships can be represented in DQV by using
the appropriate SKOS properties or by specializing the SKOS properties if more
specific semantics must be expressed. For example, ldqd:availability is related to ldqd:performance and ldqd:interlinking, whilst ldqd:semanticAccuracy is related to ldqd:timeliness,
ldqd:trustworthiness, ldqd:consistency, ldqd:syntaticValidity and ldqd:completeness.
ldqd:availability skos:related ldqd:performance ,
ldqd:interlinking .
ldqd:semanticAccuracy skos:related ldqd:timeliness ,
ldqd:trustworthiness , ldqd:consistency ,
ldqd:syntaticValidity , ldqd:completeness ,
ldqd:interlinking .
ldqd:consistency skos:related ldqd:conciseness ,
ldqd:syntaticValidity , ldqd:interoperability .
ldqd:interoperability skos:related ldqd:conciseness ,
ldqd:syntaticValidity .
ldqd:conciseness skos:related ldqd:completeness ,
ldqd:representationalConciseness .
ldqd:interpretability skos:related ldqd:versatility .
# Note: skos:related is a symmetric property, hence from every statement
# ex:subject skos:related ex:object in this example, one can infer that
# the statement ex:object skos:related ex:subject is true.
Dimensions can also be related across different categorizations.
For example, in the following, we present two
possible links between dimensions from ISO/IEC 25012 [[ISOIEC25012]]
and Zaveri et al. Here we assume that
completeness is equivalent across both classifications and
that ISO's credibility is one specific facet of trustworthiness in Zaveri et al.
(see Definition 12 in [[ZaveriEtAl]]). We pencil more such possible relationships in Annex C.
This section presents examples of metrics inspired by those reviewed in Zaveri et al. [[ZaveriEtAl]], in order to further illustrate how dqv:Metric can be instantiated. Note that they are not all specific to Linked Data quality, as some dimensions in Zaveri et al. matches the dimensions of ISO/IEC 25012 (see previous sub-section and Annex).
These examples are just some of the possible ones. They show metrics for different dimensions and kinds of dataset distributions. We might consider reorganizing examples around specific criteria (e.g., include at least a metric for each dimension, or focus on metrics for a specific kind of distribution, e.g., RDF, JSON, CSV). We might also consider to add further examples about derived metrics, multivalued metrics and extra parameters, once we have solved the remaining issues.
:downloadURLAvailabilityMetric
a dqv:Metric ;
skos:definition "It checks if dcat:downloadURL is available and if its value is
dereferenceable."@en ;
dqv:inDimension ldqd:availability ;
dqv:expectedDataType xsd:boolean
.
:sparqlAvailabilityMetric
a dqv:Metric ;
skos:definition "It checks if a void:sparqlEndpoint is specified for a dataset and
if the server responds to a SPARQL query."@en ;
dqv:inDimension ldqd:availability ;
dqv:expectedDataType xsd:boolean
.
:misreportedContentTypeMetric
a dqv:Metric ;
skos:definition "It detects whether the HTTP response contains the header field
stating the appropriate content type of the returned file, e.g. application/rdf+xml"@en ;
dqv:inDimension ldqd:availability ;
dqv:expectedDataType xsd:boolean
.
:licensingMetric
a dqv:Metric ;
skos:definition "It detects the indication of a license in a the DCAT/VoID
description or in the dataset of a license itself."@en ;
dqv:inDimension ldqd:licensing ;
dqv:expectedDataType xsd:boolean
.
:highThroughput
a dqv:Metric ;
skos:definition "It represents the maximum number of answered HTTP-requests per
second."@en ;
dqv:inDimension ldqd:performance ;
dqv:expectedDataType xsd:integer
.
:sparqlScalability
a dqv:Metric ;
skos:definition "It detects whether the time to answer an amount of ten requests
divided by ten is not longer than the time it takes to answer one request."@en ;
dqv:inDimension ldqd:performance ;
dqv:expectedDataType xsd:boolean
.
:noRDFSyntaxError
a dqv:Metric ;
skos:definition "It returns the number of syntax errors detected by an RDF
validator."@en ;
dqv:inDimension ldqd:syntacticValidity;
dqv:expectedDataType xsd:integer
.
:noJSONSyntaxError
a dqv:Metric ;
skos:definition "It returns the number of syntax errors detected by an JSON
validator."@en ;
dqv:inDimension ldqd:syntacticValidity;
dqv:expectedDataType xsd:integer
.
:populationCompletenessMetric
a dqv:Metric ;
skos:definition "Ratio between the number of objects represented in the dataset and
the number of objects expected to be represented according to the declared dataset
scope."@en ;
dqv:inDimension ldqd:completeness ;
dqv:expectedDataType xsd:double
.
R-DataMissingIncomplete: 'Publishers should indicate if
data is partially missing or if the dataset is incomplete'
R-QualityComparable: 'Data should be comparable with other
datasets'
R-Data should be complete: 'Data should be complete'
R-QualityMetrics: 'Data should be associated with a set of
documented, objective and, if available, standardized quality
metrics. This set of quality metrics may include user-defined or
domain-specific metrics.'
R-QualityOpinions: 'Subjective quality opinions on the data
should be supported'
R-GranularityLevels: 'Data available at different levels of
granularity should be accessible and modeled in a common way'
The aforementioned requirements have been further elaborated and
extended by new use cases and examples, following discussions on the
DWBP WG's mailing list, wiki pages (see here
and here),
as well as external contributions during the review process (see the
general
list of DQV issues that includes such external feedback).
Acknowledgements
The editors acknowledge the chairs of this Working Group: Hadley Beeman, Yaso Córdova, Deirdre Lee and the staff contact Phil Archer.
The editors also gratefully acknowledge the contributions made to this document by all members of the working group, specially the contributions received from Ghislain Auguste Atemezing, Carlos Laufer, Annette Greiner, Michel Dumontier, Eric Stephan.
The editors would also like to thank comments received from non-members of this working group, such as Andrea Perego, Rachel E. Heaven, Linda van den Brink, Werner Bailer, Vladimir Alexiev, Jon Blower, Guillaume Duffes, Davide Ceolin, Anisa Rula.
Changed every occurences of "Data Usage Vocabulary" into "Dataset Usage Vocabulary".
Revised the correspondences between quality dimensions in ISO/IEC 25012 and Zaveri et al. according to the latest feedback.
Correspondences between quality dimensions in ISO/IEC 25012 and Zaveri et al.
The dimensions listed in ISO/IEC 25012 [[ISOIEC25012]] and Zaveri et al. [[ZaveriEtAl]] are not disjoint.
Assuming that dimensions are expressed as instances of skos:Concept, the following table includes some of the correspondences that can be considered between these two classifications.
Dimension from Zaveri et al.
Dimension from ISO/IEC 25012
Suggested mapping relation
Availability
Availability
skos:exactMatch
Completeness
Completeness
skos:exactMatch
Consistency
Consistency
skos:exactMatch
Timeliness
Currentness
skos:exactMatch
Interoperability
Portability
skos:relatedMatch
Interoperability
Compliance
skos:relatedMatch
Semantic Accuracy
Accuracy
skos:broadMatch
Trustworthiness
Credibility
skos:narrowMatch
Trustworthiness
Traceability
skos:relatedMatch
Understandability
Understandability
skos:exactMatch
Versatility
Understandability
skos:relatedMatch
Syntactic Validity
Accuracy
skos:broadMatch
Syntactic Validity
Compliance
skos:broadMatch
Licensing
Accessibility
skos:relatedMatch
Security
Traceability
skos:relatedMatch
Security
Confidentiality
skos:relatedMatch
Performance
Efficiency
skos:exactMatch
Interlinking
Availability
skos:broadMatch
Representation-conciseness
Compliance
skos:broadMatch
Conciseness
Accuracy
skos:relatedMatch
Conciseness
Consistency
skos:relatedMatch
Defining and using parameters for metrics
Some metrics come with mandatory or optional parameters. Input parameters
in particular play a key role in describing metrics and measurements:
their values specify how a metric is applied to obtain a measurement.
E.g., when a quality metric is defined as a general procedure that can be applied to
different types of statement in the dataset, a parameter can be used to indicate the type
a given measurement has focused on. Or when various components that contribute to a metric's computation can be assigned
different weights. Section 6.10 presents examples of metrics with such parameters
(:onLanguage and :onProperty).
During the design of DQV, questions were raised about the modeling
of parameters in RDF and similar frameworks, especially about expressing that some parameters can be mandatory for measurements of some metrics
(see Issue-223).
The Working Group has postponed the issue, as we felt it was a much less mature aspect of the state of the art, and the we lacked resource
to articulate a consensual decision. This appendix presents some of the options we have considered.
We welcome readers' feedback!
Modeling parameters
The first issue is the "meta-modeling" of parameters, especially,
how properties such as :onLanguage should be represented using the modeling constructs of RDF(S) and OWL [[OWL-primer]]. One can
mint a new property :parameterValue:
:onLanguage rdfs:subPropertyOf :parameterValue .
Alternatively, one can create a class of (parameter) properties :ParameterProperty having properties like :onLanguage as instances:
:onLanguage a :ParameterProperty .
Both patterns allow to represent parameters for quality metrics.
They have their own strengths and weaknesses in terms of modeling elegance, complexity, etc. However, we could not carry a full analysis and
recommend a preferred approach. Moreover, at the time of publishing it is unclear whether using either of them
delivers crucial value to implementers interested in the basic needs presented in Section 6.10.
Especially, solutions for representing constraints using OWL axioms or Data Cube's Data Structure Definition,
as discussed in the next sub-section, do not seem to be seriously impacted by choosing either of these two meta-modeling approaches, or none.
Note finally that one may opt for a more basic solution that avoids explicit modeling parameters altogether:
hardcoding parameters in the definition of metrics. Namely, in order
to represent the measurement alternatives specific to languages (say, English or Italian)
and types of property (say, skos:prefLabel or skos:altLabel), one could "split" original
metrics like :importingForPropertyPercentage into new metrics that correspond to the relevant combination of parameters, as in the following:
:importingForPropertyPercentagePrefLabelIt a dqv:Metric ;
skos:definition "Ratio between novel Italian preferred labels
gained via skos:exactMatch links and Italian preferred labels
already in the datasets."@en
dqv:expectedDataType xsd:double ;
dqv:inDimension :completeness .
:importingForPropertyPercentagePrefLabelEn a dqv:Metric ;
skos:definition "Ratio between novel English preferred labels
gained via skos:exactMatch links and English preferred labels
already in the datasets."@en
dqv:expectedDataType xsd:double ;
dqv:inDimension :completeness .
:importingForPropertyPercentageAltLabelIt a dqv:Metric ;
skos:definition "Ratio between novel Italian alternative labels
gained via skos:exactMatch links and Italian alternative labels
already in the datasets."@en
dqv:expectedDataType xsd:double ;
dqv:inDimension :completeness .
:importingForPropertyPercentageAltLabelEn a dqv:Metric ;
skos:definition "Ratio between novel English alternative labels
gained via skos:exactMatch links and English alternative labels
already in the datasets."@en
dqv:expectedDataType xsd:double ;
dqv:inDimension :completeness .
This approach can be useful, as seen at the end of the Appendix on RDF Data Cube.
It is however much less easy to implement, should the number of parameters or the sets of their possible values grow high, or, even worse,
when the sets of possible values are not known at data modeling stage.
Parameter constraints
The second issue is the expression of constraints on parameter usage. For
example, specifying that every measurement of the metric :importingForPropertyPercentage
should be the subject of an :onLanguage statement that indicates the language considered for the measurement.
One solution is to employ the modeling features of the
Web Ontology Language OWL and create a new (sub-)class of dqv:Metric
that requires the presence of the parameter for its instances, as in this
example based on the case from Section 6.10:
:MetricWithLanguageParameter
rdfs:subClassOf dqv:Metric,
[ a owl:Restriction ;
owl:onProperty :onLanguage ;
owl:cardinality "1"^^xsd:nonNegativeInteger
] .
:linksetImportingENSKOSPrefLabel a :MetricWithLanguageParameter ;
skos:definition "Importing on English prefLabel"@en;
:onLanguage "en" .
This solution can be used for 'meta-parameters',
that is, when one considers a class of parameter-dependent metrics — where assigning a value for a parameter
allows one to define an individual metric in the class. However, this case is different from the
one of the metric "multilingual importing" in Section 6.10.
This metric indeed specifies that its measurements should be bound to specific languages. But it does not say which:
the parameter value needs to be assigned at the level of the actual measurements. Representing that would require defining a new
class of measurements, say, :MeasurementWithLanguageParameter, as follows:
# The following statements should be added to the ones from Section 6.10
:MeasurementWithLanguageParameter
rdfs:subClassOf dqv:Measurement,
[ a owl:Restriction ;
owl:onProperty :onLanguage ;
owl:cardinality "1"^^xsd:nonNegativeInteger
] .
:MetricWithLanguageParameter
rdfs:subClassOf dqv:Metric,
[ a owl:Restriction ;
owl:onProperty [ owl:inverseOf dqv:isMeasurementOf ] ;
owl:allValuesFrom :MeasurementWithLanguageParameter
] .
:importingForPropertyPercentage a :MetricWithLanguageParameter .
:measurement_exactMatchAltLabelItDataset1
dqv:isMeasurementOf :importingForPropertyPercentage .
Implementers should note that this pattern is impacted by OWL's open-world assumption. The class definitions above do not lead to constraints in the
"traditional" understanding! I.e., if an OWL reasoner finds a measurement of :importingForPropertyPercentage without an :onProperty statement, it will not raise a formal inconsistency error.
It will instead just assume that the statement must have been asserted in another RDF graph elsewhere. Other
approaches may be followed to palliate this. The RDF Data Cube vocabulary offers a Data Structure Definition mechanism, which we
exemplify in the Appendix on RDF Data Cube.
This solution allows a Data Cube validation tool to flag a language-less measurement of
:importingForPropertyPercentage as incomplete. However, as hinted in this Appendix,
Data Cube's Data Structures are harder to apply when quality assessments
of different types are mixed together. A general constraint expression language like the coming SHACL [[SHACL]]
may provide an appropriate solution. However, SHACL is still under development at the time of writing this document.
Compatibility with RDF Data Cube
The RDF Data Cube vocabulary [[Vocab-Data-Cube]] provides a means to represent multi-dimension data, including statistics. Measurements represented in DQV can fit this approach, and there might be many benefits in representing them in a way compatible with Data cube prescriptions — starting with the possibility to load measurement data in visualization or processing tools compatible with Data Cube.
In DQV, instances of dqv:QualityMeasurement are also instances of Data Cube's qb:Observation. A dataset (RDF graph) of DQV measurements can be made further compatible with Data Cube by specifying an appropriate (Data Cube) Data Structure Definition, following a pattern introduced by the daQ vocabulary [[DaQ-RDFCUBE]]. The following example is aimed at representing measurements for the metrics capturing the quality of a linkset presented in Section 6.10:
:linksetQualityMeasurements a dqv:QualityMeasurementDataset ;
qb:structure ex:dsd .
ex:dsd a qb:DataStructureDefinition ;
## Expressing Data Cube dimensions
qb:component [ qb:dimension dqv:isMeasurementOf ;
qb:order 1
] ;
qb:component [ qb:dimension dqv:computedOn ;
qb:order 2
] ;
qb:component [ qb:dimension dcterms:date ;
qb:order 3
] ;
## Expressing the Data Cube measure
qb:component [ qb:measure dqv:value ; ] ;
## Expressing the Data Cube attribute (here, unit of measurement)
qb:component [ qb:attribute sdmx-attribute:unitMeasure ;
qb:componentRequired false ;
qb:componentAttachment qb:DataSet
] .
Allowing data publishers to use the properties dqv:isMeasurementOf and dqv:computedOn in such data structures is actually the reason why they have been defined as instances of qb:DimensionProperty>. The following statements represent the linking of actual measurements in Section 6.10 to the dataset specified by the Data Structure Definition:
DQV users should be aware that applying Data Cube Data Structure Definitions to their quality information datasets has a broad impact on the possible content of these. All resources that are said to be in the dataset (using the qb:dataSet property) are indeed expected to have the components defined as mandatory in the data structure!
RDF Data Cube also states that "no two qb:Observations in the same qb:DataSet may have the same value for all dimensions". This integrity constraint implies that it is not allowed to have two distinct measurements for the same metric, resource and date. Metrics depending on parameters such as the ones from Section 6.10 shall be used with extra care so as to adhere to this constraint: data publishers will probably need to represent quality measurements for the same resource and date, but which are obtained by applying distinct parameters.
Adopters of DQV and Data Cube can consider two modelling options for addressing this:
To extend the above Data Structure with the desired parameters;
To define a new Metric for each combination of the parameters values.
Let's apply the first option to the example of Section 6.10.
The “Multilingual importing” metric there requires to manage two parameters, :onProperty and :onLanguage, which gave raise to one instance of owl:DataProperty each. These parameters can be also expressed as instances qb:DimensionProperty and the above Data Structure extended as follows:
## Adding a new type to the parameter properties
:onLanguage a qb:DimensionProperty .
:onProperty a qb:DimensionProperty .
## Extending the structure of daq:dsq with two new dimensions
ex:dsd qb:component [ qb:dimension :onProperty ;
qb:order 4
] ;
qb:component [ qb:dimension :onLanguage ;
qb:order 5
] .
Alternatively, data publishers can keep to the original Data Structure. But in this case
they should be prepared to not use the parameter properties introduced in 6.10.
Instead, they could use the "split" metrics presented at the end of the
Appendix on parameters. This solution is however not always applicable,
as this Appendix shows it.
