As any modeller will know, there are pieces of information or knowledge that can be represented in different ways. For instance, representing ‘marriage’ as class or as a ‘married to’ relationship, adding ‘address’ as an attribute or a class in one’s model, and whether ‘employee’ will be positioned as a subclass of ‘person’ or as a role that ‘person’ plays. In some cases, there a good ontological arguments to represent it in one way or the other, in other cases, that’s less clear, and in yet other cases, efficiency is king so that the most compact way of representing it is favoured. This leads to different design decisions in ontologies, which hampers ontology reuse and alignment and affects other tasks, such as evaluating competency questions over the ontology and verbalising ontologies.
When such choices are made consistently throughout the ontology, one may consider this to be a modelling style or representation style. If one then knows which style an ontology is in, it would simplify use and reuse of the ontology. But what exactly is a representation style?
While examples are easy to come by, shedding light on that intuitive notion turned out to be harder than it looked like. My co-author Pablo Fillottrani and I tried to disentangle it nonetheless, by characterising the inherent features and the dimensions by which a style may differ. This resulted in 28 different traits for the 10 identified dimensions. For instance, the dimension “modular vs. monolithic” has three possible options: 1) ‘Monolithic’, where the ontology is stored in one file (no imports or mergers); 2) ‘Modular, external’, where at least one ontology is imported or merged, and it kept its URI (e.g., importing DOLCE into one’s domain ontology, not re-creating it there); 3) ‘Modular, internal’, where there’s at least one ontology import that’s based on having carved up the domain in the sense of decomposition of the domain (e.g., dividing up a domain into pizzas and drinks at pizzerias). Other dimensions include, among others, the granularity of relations (many of few), how the hierarchy looks like, and attributes/data properties.
We tried to “eat our own dogfood” and applied the dimensions and traits to a set of 30 ontologies. This showed that it is feasible to do, although we needed two rounds to get to that stage—after the first round of parallel annotation, it turned out we had interpreted a few traits differently, and needed to refine the number of traits and be more precise in their descriptions (which we did). Perhaps unsurprising, some tendencies were observed, and we could identify three easily recognisable types of ontologies because most ontologies had clearly one or the other trait and similar values for sets of trait. Of course, there were also ontologies that were inherently “mixed” in the sense of having applied different and conflicting design decisions within the same ontology, or even included two choices. Coding up the results, we generated two spider diagrams that visualise that difference. Here’s one:
Details of the dimensions, traits, set-up and results of the evaluation, and discussion thereof have been published this week  and we’ll present it next month at the 1st Iberoamerican Conference on Knowledge Graphs and Semantic Web (KGSWC’19), in Villa Clara, Cuba, alongside 13 other papers on ontologies. I’m looking forward to it!
 Keet, C.M., Fillottrani, P.R.. Dimensions Affecting Representation Styles in Ontologies. 1st Iberoamerican conference on Knowledge Graphs and Semantic Web (KGSWC’19). Springer CCIS vol 1029, 186-200. 24-28 June 2019, Villa Clara, Cuba. Paper at Springer
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