Part of a Vision for Information Interoperability on the Web
Wikipedia is arguably the most important information source yet invented for natural language processing (NLP) and artificial intelligence, in addition to its role as humanity’s largest encyclopedia. Wikipedia is the principal information source for such prominent services as IBM’s Watson , Freebase , the Google Knowledge Graph , Apple’s Siri , YAGO , and DBpedia , the core reference structure for linked open data . Wikipedia information has assumed a prominent role in NLP applications in word sense disambiguation, named entity recognition, co-reference resolution, and multi-lingual alignments; in information retrieval in query expansion, multi-lingual retrieval, question answering, entity ranking, text categorization, and topic indexing; and in semantic applications in topic extraction, relation extraction, entity extraction, entity typing, semantic relatedness, and ontology building .
The massive size of Wikipedia — with more than 26 million articles across 250 different language versions [9,10] — makes it a rich resource for reference entities and concepts. Structural features of Wikipedia that help to inform the understanding of relationships and connections include articles (and their embedded entities and concepts), article abstracts, article titles, infoboxes, redirects, internal and external links, editing histories, categories (in part), discussion pages, disambiguation pages, images and associated metadata, templates, tables, and pages of lists, not to mention Wikipedia as a whole being used as a corpus or graph . It is no wonder that Wikipedia is referenced in about 665,000 academic articles and books . And all of this occurs in a phenomenon that is not yet 15 years old!
Wikipedia is unparalleled as a resource for mining these resources of structure, concepts and entities. But, and here is the challenge, Wikipedia is never itself used as a computable knowledge base. It is a resource for other knowledge systems, but not a coherent knowledge base unto itself. Wikipedia feeds other useful knowledge bases, but does not play those roles alone. Why this is and how it can be remedied is the subject of this article.
Three Basic Problems
Wikipedia has been cited for three weaknesses relevant to its role as a knowledge base. The first is that its coverage is imbalanced. Various studies have evaluated the scope of Wikipedia [13, 14, 15, among many] and have found areas of popular culture such as games, movies, music and actors to be over-represented, while areas of philosophy, technology, academics and history, to be under-represented. While still perhaps true in terms of absolute numbers of articles, the actual domain coverage has been improving in recent years.
The second Wikipedia problem is incompleteness. Wikipedia tends to be spotty in terms of providing complete and equal representation in populating certain categories (or classes) with articles (instances). It also tends to be incomplete in how well embedded or structured various articles may be. An example of the representation problem is in economy or commerce and the coverage of companies or products. The notability criterion  is a tricky one here; some companies or products with seemingly equivalent notability get listed, while others do not. Another example is the kingdom of life where some life forms are extremely well represented, while others are not. The incompleteness of structure relates to which articles or entire categories have infoboxes or ones that are well populated, as well as how category assignments are incomplete or inconsistent. The existence of “stub” articles is one evidence for such incompleteness. As Wikipedia has gotten more structured and complicated, the number of active editors has declined. The growing use of bots, however, is often compensating for this and in some cases bringing better consistency and equivalent treatment [17,18].
But the biggest problem of Wikipedia has been its category structure. Categories were not part of the original design, but were added to Wikipedia in 2004. Various reviewers have likened Wikipedia more to a thesaurus than a classification scheme , others that it is different than classical knowledge organization systems in that it has no specified root or hierarchy . This improved a wee bit from 2006 to 2010, when the main Wikipedia topics were organized according to top-level and main topics . Still, typical commentaries point to the fact that Wikipedia’s category structure is “noisy, ill-formed, and difficult to make sense of” . Its crowdsourced nature has led to various direct and indirect cycles in portions of the category structure . All of these problems lead to the inability to do traditional reasoning or inference over the Wikipedia category graph .
Besides these lacks of computability, the Wikipedia graph is bloated with “artificial” categories (see further below) that just add noise to trying to understand or navigate the Wikipedia category structure. In short, while Wikipedia is a goldmine of resources and partial structure, its organization is incoherent at a global level, and it is unable to support reasoning and other tasks that might be expected from a truly functional knowledge base.
The real shame — but also the real opportunity — is that this lack of coherency makes it more difficult to validate and improve the information already in Wikipedia. So, there are both external reasons of linkage and internal reasons of improved authority for which it is desirable to shape Wikipedia into a true knowledge base.
Efforts to Recast Wikipedia
These faults are not unrecognized and the prospect of better leverage from Wikipedia has stimulated many efforts. Gazing inward, it is not uncommon to find efforts that attempt to clean up the existing Wikipedia structure , or various attempts to use the content of Wikipedia article categories  to re-constitute new taxonomies  or concept networks . Clean up appears essential, and is a relative constant in other attempts to recast Wikipedia .
The choice of Wikipedia’s founders to make its full content available electronically for free and without restriction was a masterstroke. This has stimulated many to grab the Wikipedia content and to recast it in other ways. One of the first, and most successful, was DBpedia, with an emphasis on making (much of) Wikipedia available in RDF and linked data. DBpedia emphasized the structured content of Wikipedia’s infoboxes and eventually derived a typology of entities and their properties expressed as the DBpedia ontology . It is not hyperbole to state that DBpedia nucleated the entire linked data phenomenon .
The key insight of YAGO  was the recognition that the resource richness of Wikipedia lacked a unifying structure, with WordNet chosen as the replacement organizing framework. Also, by patterned analysis of Wikipedia’s article titles structure, YAGO was able to infer and select many attribute relationships between entities. This enabled YAGO to posit what, in essence, was a much-expanded category structure for Wikipedia expressed as predicates. Many other efforts have also chosen WordNet as their organizing framework for Wikipedia [31,32].
Freebase , itself another attempt to use crowdsourcing with explicit attention to structured data, struggled in its early years until it embraced and incorporated Wikipedia. That marked the take-off point for Freebase, which was later acquired by Google to form the backbone of its knowledge graph. Freebase is now being shut down with its assets being transferred to Wikidata.
Wikidata  is itself an interesting case of how the Wikipedia model is being expanded. Wikidata, a sister project to Wikipedia under the Wikimedia banner, takes as its starting point the structured data about entities evident in Wikipedia infoboxes. Rather than extracting and cleaning that entity information as DBpedia does, the role of Wikidata is to be the multilingual source for all entities feeding the Wikimedia network, including Wikipedia. The approach leads to more uniformity and consistency, and provides a central Wikimedia access point for structured data. However, somewhat akin to Wikipedia, Wikidata also has struggled to find an appropriate typology (or ontology) for its millions of entities .
Other approaches to the Wikipedia classification challenge have been to map — or “express” — Wikipedia articles in relation to established external vocabularies or structures, such as the Library of Congress Classification , Library of Congress Subject Headings [23, 36], Universal Decimal classification (UDC) , Cyc  or UMBEL , among others. The idea here is that accepted organizational schemes provide more coherence than the Wikipedia category structure, with sometimes additional benefits as well.
Though not complete topical recastings, certain aspects of Wikipedia have also proven their usefulness for general knowledge acquisition. Using article (concept or entity) content can inform topical tagging using explicit semantic analysis (ESA) , automatic topic identification , information extraction  or a myriad of others.
Making a Natural Wikipedia Category Scheme
Whether “cleaned” or recasted, taking the existing Wikipedia structure in its existing form is problematic. Though some category cleaning sometimes takes place with some of these uses of Wikipedia, that is not uniformly nor universally so. The cleaning that does take place is often limited to administrative categories (relating to internal Wikipedia conventions or management). However, other Wikipedia conventions (such as lists) and the proliferation of user-generated “artificial” categories actually represent the bulk of the total number of categories.
Charles S. Peirce was the first, by my reading, who looked at the question of “natural classes,” which are now sometimes contraposed against what are called “artificial classes” (we tend to use the term “compound” classes instead). A “natural class” is a set with members that share the same set of attributes, though with different values (such as differences in age or hair color for humans, for example). Some of those attributes are also more essential to define the “type” of that class (such as humans being warm-blooded with live births and hair and use of symbolic languages). Artificial classes tend to only add one or a few shared attributes, and do not reflect the essence of the type .
“Compound” (or artificial) categories (such as Films directed by Pedro Almodóvar or Ambassadors of the United States to Mexico) are not “natural” categories, and including them in a logical evaluation only acts to confuse attributes from classification. To be sure, such existing categories should be decomposed into their attribute and concept components, but should not be included in constructing a schema of the domain.
“Artificial” categories may be identified in the Wikipedia category structure by both syntactical and heuristic signals. One syntactical rule is to look for the head of a title; one heuristic signal is to select out any category with prepositions. Across all rules, “compound” categories actually account for most of what is removed in order to produce “cleaned” categories.
We can combine these thoughts to show what a “cleaned” version of the Wikipedia category structure might look like. The 12/15/10 column in the table below reflects the approach used for determining the candidates for SuperTypes in the UMBEL ontology, last analyzed in 2010 . The second column is from a current effort mapping Wikipedia to Cyc :
Two implications can be drawn from this table. First, without cleaning, there is considerable “noise” in the Wikipedia category structure, equivalent to about half to two-thirds of all categories. Without cleaning these categories, any analysis or classification that ensues is fighting unnecessary noise and has likely introduced substantial assignment errors. Second, approaches, assumptions and how filters get sequenced differ between “cleaning” attempts, which both makes comparability a challenge but also represents areas for discussion and testing to derive best practices. This lack of comparability due to differences in staging Wikipedia for analysis makes it difficult to draw comparisons between different studies and approaches. One study is not necessarily relatable to other studies.
Today, in chaotic and uncoordinated ways, we see Wikipedia feeding much analysis through partial aspects of its structure and supplying many reference concepts and entities. But each analysis is done for different purposes using different bases; they are thus incompatible. Coherency, usability and insight suffer. Any prior efforts to map to or use Wikipedia categories that do not remove these artificial categories only introduce noise and are therefore likely to be in substantial error.
Benefits of a Reference Knowledge Base
If we could overcome these shortcomings by taking the steps to make Wikipedia a true reference knowledge base, what might the benefits be? Or, said another way, why should we care?
One benefit is that reference structures of any kind provide a focus, by definition, of common or canonical referents. This commonality leads to better defined, better understood and more widely used referents. Common referents become a kind of common vocabulary for the space, upon which other vocabularies and datasets can reference. A common language, of sorts, can begin to emerge.
Reference structures also provide a grounding, a spoke-and-hub design , that leads to an efficient basis for external vocabularies and datasets to refer to one another. Of course, any direct mapping can provide a means to relate this information, but such pairwise mappings are not scalable nor efficient. In a spoke-and-hub design, the number of mappings required goes down significantly with the number of datasets or items requiring mapping. The spoke-and-hub design, for example, is at the heart of such disciplines as master data management.
Another benefit of common reference structures is that they provide a common target for the development of tools and best practices. These kinds of “network effects” lead to still further tooling and practices. Thus, while we see literally tens of thousands of academic papers and approaches leveraging Wikipedia in one way or another, we see little of a practice or a community that has been built around it as a knowledge base. It is as if we are still looking a bit at the shadow of Wikipedia and its possible role, a chimera for its potential as a true knowledge base.
But the ultimate benefit of Wikipedia as a reference knowledge base will reside in its computability. When we can reason over Wikipedia’s content, use it for testing and analyzing assertions or new facts, when its coherent organization can be applied to such tasks as informing how to map and interoperate data together or remaking whole legacy applications such as enterprise information integration or MDM, all of which in cross-lingual ways, we will finally see the realization of Wikipedia’s inherent potential. And, as these latent capabilities get exploited, we will see supporting knowledge sources such as Wikidata also get pulled into the ecosystem.
Seven Requirements for a Computable Knowledge Base
So, if we buy into the benefits of a computable Wikipedia — or any other useful knowledge source for that matter — what are the guideposts for doing so? How do we assess the gaps and then fill them?
The importance of working with a “clean” version of the Wikipedia structure is obvious, yet ultimately there are higher-order requirements for what it takes, in our view, to become a “true” reference knowledge base. By our definition, such KBs have these aspects:
- Coherent — does it hold together conceptually, logically, does it make sense? Either internally via consistency tests and such, or externally via testing against known facts and knowledge, the structure of the knowledge base should be defensible and meet the “common sense” test
- Comprehensive — does the knowledge base have the scope of domains to which it is likely to interact? For a Web reference, the KB need not be global, but be relevant to an important domain of discourse. The biomedical domain, and its constituent and biological sub-domains, is an example. Something like Wikipedia represents a more “global” domain, and is thus central to the idea
- Referencable — is the knowledge source authoritative? does it use URIs for referencing its objects?
- Open Standards — which also implies, does it meet open standards? Open standards, by virtue of their decision processes, represent well-reasoned bases. Open standards are also easier to interoperate with and have more tooling available
- Computable — the combination of the above can lead to a KB structure that supports reasoning, inference, set selection, relations, attributes, datatypes, and filtering and retrieval. These aspects make the KB “computable” , the threshold qualifier for a “true” knowledge base
- Testable — but now, once the KBs are computable, they are also testable. That means the entire KB structure may be tested, verified, validated, scored, and evaluated
- Multi-lingual — if not already multi-lingual, does it have a structure (such as ID v label-based) that supports multiple languages? Is there attention paid to encoding and transfer standards so as to promote consumption and use of the KB data? Multi-linguality may sound like icing on the cake, but it represents the next phase of bringing structure to the question of how to better identify, discern, and disambiguate information.
Wikipedia, and other publicly available knowledge sources , already fulfill many of these requirements. With focused attention, any current reference source should be able to be lifted to meet these seven major requirements.
Outlines of a General Staging Pipeline
OK, then: what might such a KB processing (or “lifting”) approach look like?
Well, the first point is that it should be a pipeline. It is important to be able to swap in and out various options at multiple points from input to desired output. Then, because there are disparate sources and different formats to accommodate, it is also important to use canonical syntaxes and standards for expressing the products and specifications at the various steps along that pipeline.
The very notion of pipeline implies workflows, which are the actual drivers for how the pipeline should be designed. Key workflow steps include:
- Clean the input sources
- Express the sources in a canonical form 
- Identify and extract concepts
- Map the structure to KB concepts
- Identify and extract entities
- Identify and extract relations
- Type the entities, concepts, and relations
- Extract attributes and values for identified entities
- Test these against the existing KB
- Update reference structures, including placement of the new assertions, as appropriate
- Characterize and log to files
- Commit to the KB
- Rinse, repeat.
Much information gets processed in these pipelines, and the underlying sources update frequently. Thus, the pipelines themselves need to be performant and based on solid code. Automation, within the demanding bounds of quality, is also an essential condition to be scalable. Improving on that is a process, not a state.
Time to Make Some Sausage
Most of these observations are really not new or innovative [39,50]. Possibly what is new is to articulate the situation for major reference sources on the Web, and to then analyze and propose how to process them in the service of information interoperability.
Because, you see, we’re still at the very, very earliest phases of how the Internet is changing the abilities to gather, understand, and represent the information in our world. We’re about ready to embark on the next stage in that journey.