Linked Data Quality Assessment with RDFUnit
RDFUnit [10–12] is a framework for test-driven Linked Data quality assessment, which is inspired by test-driven software development. A key principle of test-driven software development is to start the development with the implementation of automated test-methods before the actual functionality is implemented. Compared to software source code testing, where test cases have to be implemented largely manually or with limited programmatic support, the situation for Linked Data quality testing is slightly more advantageous. On the Data Web we have a uniﬁed data model – RDF – which is the basis for both, data and ontologies. RDFUnit exploits the RDF data model by devising a pattern-based approach for the data quality tests of knowledge bases. Ontologies, vocabularies and knowledge bases can be accompanied by a number of test cases, which help to ensure a basic level of quality. This is achieved by employing SPARQL query templates, which are instantiated into concrete quality test SPARQL queries. We provide a comprehensive library of quality test patterns, which can be instantiated for rapid development of more test cases. Once test cases are deﬁned for a certain vocabulary, they can be applied to all datasets reusing elements of this vocabulary. Test cases can be re-executed whenever the data is altered. Due to the modularity of the approach, where test cases are bound to certain vocabulary elements, test cases for newly emerging datasets, which reuse existing vocabularies can be easily derived.
RDFUnit is capable of performing quality assessments with only a minimal amount of manual user intervention and is easily applicable to large datasets. Other tools like the TopBraid Composer  use the SPARQL Inferencing Notation
Fig. 11. Flowchart showing the test-driven data quality methodology. The left part displays the input sources of our pattern library. In the middle part the diﬀerent ways of pattern instantiation are shown which lead to the data quality test cases on the right.
(SPIN) to deﬁne SPARQL queries for quality assessment. However, RDFUnit utilizes an own SPARQL template notation, which better suits our methodology. An overview of the methodology is depicted in Fig. 11.
Analysis of Link Validity
Web Linkage Validator
With the integration of data into the LOD cloud, it is essential that links between datasets are discoverable as well as eﬃciently and correctly assessed. The Web Linkage Validator is a web-based tool that allows for knowledge base owners to improve their data with respect to linkage and to assess their linked data for integration with the LOD cloud.
The goal is to provide a tool to the LOD2 stack to aid in assessing links between LOD datasets. It analyses the links between entities that a dataset has as well as links to entities from other datasets. It will help knowledge base users in improving the quality of links of their datasets.
The Web Linkage Validator's assessment is based on the concept of a data graph summary [3, 4]. A data graph summary is a concise representation of the RDF data graph and is composed of the structural elements, i.e., class and property. The information it contains, such as RDF class and predicate, usage frequency, provenance and linkage, are the basis for suggesting to knowledge base owners ways in which they may create or improve the links within their datasets and with other external datasets.