# A Framework for the Evaluation of the Cultural Heritage Information Ontology
## 4 Discussion and Conclusions
The vastness of the heritage domain, and in particular of built heritage, coupled with the growing necessity for a reliable domain ontology and the existence of several core ontologies that can be reused, justified the inclusion of the Interoperability characteristic (metric) in the evaluation of the cultural heritage information ontology. For the calculation of the OQuaRE complex metrics, a new software framework for ontology evaluation was developed. The software uses Apache Jena Ontology API ([[http://jena.apache.org/](http://jena.apache.org/)]([http://jena.apache.org/](http://jena.apache.org/)), accessed on 10 January 2022). All OQuaRE quality metrics for the CIDOC CRM are ranked between 2.6 and 3.83, and the total quality average is 3.31, which ranks it as a minimally acceptable ontology according to the established scoring table. The lowest metric scores are for Reliability (2.6) and Functional Adequacy (2.5). Reliability is defined as \"capability of an ontology to maintain its level of performance under stated conditions for a given period of time\" and is made up of _WMCOnto2_, _DITOnto_, _NOMOnto_, and _LCOMOnto_. _DITOnto_ (depth of subsumption hierarchy) expresses the length of the largest path from _owl:Thing_ to a leaf class. As already stated, the CIDOC CRM maintains its own high-level and low-level domain concepts that significantly contribute to the low _DITOnto_ score of 1 because of the 10 levels of subclass hierarchy (including _owl:Thing_). _LCOMOnto_ (lack of cohesion in methods) expresses the separation of responsibilities and independence of components of ontologies and is measured as the total length of all paths from leaves to _owl:Thing_ divided by number of paths. _LCOMOnto_ has a value of 6.86. Simulation shows that separation of the two hierarchy levels containing high-level concepts (_'CRM Entity'_, _Dimension_, _'Persistent Item'_, _Place_, _'Spacetime Volume'_, _'Temporal Entity'_, _Time-Span_) would increase Reliability to 3.8 and total ontology quality to 3.73. Therefore, the recommendation is to flatten the ontology. The Structural characteristic is influenced by the Tangledness sub-characteristic calculated by _TMOnto2_ (2). An increase in the value has a negative effect on the total evaluation. For the _TMOnto2_, the formula was improved, and it shows that its value cannot be lower than 2, which contradicts published work showing an example with _TMOnto2_ < 2 [57]. Functional adequacy characteristic is \"the capability of the ontologies to provide concrete functions\" to applications that consume relations and properties from an ontology. The score (2.92) is low because of the low scores for _RROnto_ (2), _AROnto_ (1), and _CROnto_ (1). _RROnto_ (relationship richness) is calculated as number of subconcepts (_rdfs:subClassOf_) divided by the sum of subconcepts (_rdfs:subClassOf_) plus object and data properties (_owl:ObjectProperty_, _owl:DatatypeProperty_) of the concepts. The CIDOC CRM has 176 _subclassOf_ relationships usages and 286 object and data properties. As an example, Figure 3 explains that the concept _E81_Transformation_ contributes two relationships (_rdfs:subClassOf_) and four object properties (2 _rdfs:domain_ and 2 _rdfs:range_). The great number of properties creates an imbalance with the number of relationships, which scores very low for \(RROnto\). _CROnto_ measures the mean number of individuals per class, but CIDOC CRM has no individuals; therefore, the score is 0. _AROnto_ measures the number of restrictions of the ontology per class. Since it is not exactly clear, we assume that restrictions in this context are _owl:Restriction_ predicates in ontology. They are not defined in the CIDOC CRM; therefore, the score is 0. Structural quality of the CIDOC CRM is low (3.25), although the metric _ANOnto_ is high because it counts occurrences for _rdfs:comment_ and _rdfs:label_ in seven different languages. Moreover, in the CIDOC CRM, there are 15 concepts with exactly two direct superclasses.