Data Quality

Wouter Beek (

Part I: What is the problem>?

After 15 years semantic data cannot be…

  • found
  • read
  • queried
  • reasoned over

Many PhD students' worse nightmare.

Problem 1: Most data cannot be found

SotA findability comparable to 1995 Yahoo! index

Problem 2: Most data cannot be read

E.g., Freebase <10% syntactically correct.

Current approaches are inherently slow: standards, guidelines, best practices, tools, education.

This takes decades!

Why is data dirty?

  • Character encoding issues
  • Socket errors
  • Protocol errors
  • Corrupted archives
  • Authentication problems
  • Syntax errors
  • Wrong metadata
  • Lexical form ↛ value
  • Non-canonical lexical form
  • Logically inconsistent

Problem 3: Most data cannot be queried

  • Few SPARQL endpoints with high availability.
  • Web-scale ‘Follow Your Nose’ = DoS attack
  • SPARQL endpoints with high availability use non-standardized pagination.
  • No streaming
  • No anytime behavior

Problem 4: Most data cannot be reasoned over

  • SPARQL Entailment Regimes are not standardized.
  • Some entailment results cannot be expressed in RDF.
  • Existing reasoners give incomplete results.
  • Web-scale reasoning is only performed in the lab
  • Real-world reasoning immediately goes ex falso quodlibet

Part II: How to solve this?

Beek & Rietveld & Bazoobandi & Wielemaker & Schlobach. 2014. “LOD laundromat: A Uniform Way of Publishing Other People’s Dirty Data” ISWC.


Wielemaker & Beek & Hildebrand & Van Ossenbruggen. 2016. ‘ClioPatria: A SWI-Prolog Infrastructure for the Semantic Web’ in Semantic Web Journal (
Header Dictionary Triples (HDT)
Fernández & Martínez-Prieto & Gutiérrez & Polleres & Arias. 2013. ‘Binary RDF representation for publication and exchange(HDT)’ in Web Semantics: Science, Services and Agents on the World Wide Web, Vol. 19, p. 22-41 (

Data access (1/2)

Data access (2/2)

Rietveld & Verborgh & Beek & Vander Sande & Schlobach. 2015. “Linked Data-as-a-Service: The Semantic Web Redeployed” ESWC.

Semantic Web layer cake

Alt. Semantic Web layer cake

Beek & Rietveld & Schlobach & Van Harmelen. 2016. “LOD Laundromat: Why the Semantic Web Needs Centralization (Even If We Don't Like It)”, IEEE Internet Computing 20 (2) p.78-81.

Literally Better: Cleaning literals

Why literals?

  • Concise notation for infinite value spaces.
  • Encoding of linguistic/text-based content.

W. Beek & F. Ilievski & J. Debattista & S. Schlobach. “Literally Better: Analyzing and Improving the Quality of Literals” (under submission)

Benefits of high-quality literals

  • Efficient computation through canonicity
  • Data enrichment by improved instance matching
  • User eXperience: language preference, “value labeling”
  • Improve NLP tasks with background knowledge

Thank you for your attention!