1. Semantic Web for the Working Ontologist Dean Allemang Jim Hendler SNU IDB laboratory

2. Semantic Web for the Working OntologistDean Allemang, Jim Hendler Working Ontology Contents 2 ■ Chapter 1 What is the Semantic Web? ■ Chapter 2 Semantic Modeling ■ Chapter 3 RDF-The Basis of the semantic Web ■ Chapter 4 Semantic Web Application Architecture ■ Chapter 5 RDF and Inferencing ■ Chapter 6 RDF Schema ■ Chapter 7 RDFS-Plus ■ Chapter 8 Using RDFS-Plus in the Wild ■ Chapter 9 Basic OWL ■ Chapter 10 Counting and Sets in OWL ■ Chapter 11 Using OWL in the Wild ■ Chapter 12 Good and Bad Modeling Practices ■ Chapter 13 OWL Levels and Logic

3. Semantic Web for the Working OntologistDean Allemang, Jim Hendler Chapter 12 Good and Bad Modeling Practices ■ Getting Started ■ Modeling For Reuse ■ Common Modeling Errors 3

4. Semantic Web for the Working OntologistDean Allemang, Jim Hendler Getting Started [1/3] ■ How can you start the construction of a useful semantic model? ■ Two issues for the modeler: How do I express my intended purpose for a model? How do I determine whether a model satisfies some purpose? 4

5. Semantic Web for the Working OntologistDean Allemang, Jim Hendler Getting Started [2/3] ■ Know What You Want One common practice for semantic models usually starts with the notion of “competency questions” 1. Begin the modeling process by determining what questions the model will need to answer 2. Then construct the model so that these questions can be answered 3. And, to the extent possible, model no further than necessary to answer them 5

6. Semantic Web for the Working OntologistDean Allemang, Jim Hendler Getting Started [3/3] ■ Inference Is Key We can determine a model’s answer to a particular question (or query) through an analysis of inferencing The consistency of the answers to the various questions is expressed & maintained through inferencing 6

7. Semantic Web for the Working OntologistDean Allemang, Jim Hendler Modeling For Reuse [1/6] ■ One of the principle drivers in the creation of a semantic model is that It will be used by someone other than its designer in a new context Consider the challenges the people using your model might face! 7

8. Semantic Web for the Working OntologistDean Allemang, Jim Hendler Modeling For Reuse [2/6] ■ Insightful Names vs. Wishful Names Keeping in mind that your model will be read by others is always good practice A misleading example ▶ :cheese skos:broader :dairy Judicious use of rdfs:label can alleviate this issue ▶ skos:broader rdfs:label “has broader term” 8

9. Semantic Web for the Working OntologistDean Allemang, Jim Hendler Modeling For Reuse [3/6] ■ De facto standard ways to name entities on the Semantic Web Name resources in CamelCase ▶ rdfs:subClassOf, owl:InverseFunctionalProperty Start class names with capital letters ▶ owl:Restriction, owl:Class Start property names with lowercase letters ▶ rdfs:subClassOf, owl:inverseOf Start individual names with capital letters ▶ lit:Shakespeare, ship:Berengaria Name classes with singular nouns ▶ owl:DatatypeProperty, lit:Playwright 9

10. Semantic Web for the Working OntologistDean Allemang, Jim Hendler Modeling For Reuse [4/6] ■ Keeping Track of Classes & Individuals One of the greatest challenges when designing a semantic model is, Determining when something should be modeled as a class or an individual 10

11. Semantic Web for the Working OntologistDean Allemang, Jim Hendler Modeling For Reuse [5/6] ■ Keeping Track of Classes & Individuals Some general guidelines ▶ 1. The simple observation that classes can be seen as a set of instances − If something is modeled as a class, then there should be at least be a possibility that the class might have instances ▶ 2. The properties that describe the thing to be modeled − We know in general that a play has an author, a first performance date, & one or more protagonists − But we know specifically about The Tempest that it was written by William Shakespeare, was performed in 1611, & has protagonist Prospero -> The Tempest : instance play: class 11

12. Semantic Web for the Working OntologistDean Allemang, Jim Hendler Modeling For Reuse [6/6] ■ Model Testing How can we test a model? We can determine ▶ whether it maintains consistent answers to possible competency questions from multiple sources ▶ test cases for the model We’ll refer generally to model tests-ways you can determine if the model satisfies its intent 12

13. Semantic Web for the Working OntologistDean Allemang, Jim Hendler Common Modeling Errors [1/19] ■ We can’t say that the models are strictly erroneous due to the AAA slogan ■ But we can say that they do not accomplish the reuse goals of a semantic model ■ Antipatterns Common pitfalls of beginning modelers 13

14. Semantic Web for the Working OntologistDean Allemang, Jim Hendler Common Modeling Errors [2/19] - Rampant Classism (Antipattern) ■ A common reaction to the difficult distinction between classes & instances is simply to define everything as a class Begin by creating a class with the name of an important, central concept Then, extend it by creating more classes whose names indicate concepts that are related to the original 14

15. Semantic Web for the Working OntologistDean Allemang, Jim Hendler Common Modeling Errors [3/19] - Rampant Classism (Antipattern) ■ E.g. modeling Shakespeare & his works Defined like the following: And perhaps even This seems to makes sense 15 - Classes Shakespeare Plays Poems Poets Playwrights TheTempest

16. Semantic Web for the Working OntologistDean Allemang, Jim Hendler Common Modeling Errors [4/19] - Rampant Classism (Antipattern) 16

17. Semantic Web for the Working OntologistDean Allemang, Jim Hendler Common Modeling Errors [4/19] - Rampant Classism (Antipattern) ■ Testing with competency questions “Who wrote The Tempest ?” & “What type of things writes plays?” ▶ -> The answer is available directly in the model “What type of thing wrote Elizabethan plays?” ▶ -> None of the inference patterns we have learned apply here and, in particular, there is no inference of the form 17

18. Semantic Web for the Working OntologistDean Allemang, Jim Hendler Common Modeling Errors [5/19] - Rampant Classism (Antipattern) ■ Another test criterion Whether it can distinguish between plays & types of plays ▶ The Tempest is a play Elizabethan play & and modern play are types of plays ▶ -> The model cannot distinguish these two cases 18

19. Semantic Web for the Working OntologistDean Allemang, Jim Hendler Common Modeling Errors [6/19] - Exclusivity (Antipattern) ■ The members of a subclass are necessarily members of a superclass ■ The fallacy of exclusivity to assume that the only candidates for membership in a subclass are those things that are already known to be members of the superclass 19

20. Semantic Web for the Working OntologistDean Allemang, Jim Hendler Common Modeling Errors [7/19] - Exclusivity (Antipattern) ■ Example We have a class called City & a subclass OceanPort We might have a number of members of the class City 20

21. Semantic Web for the Working OntologistDean Allemang, Jim Hendler Common Modeling Errors [8/19] - Exclusivity (Antipattern) ■ Example (cont’d) We assume that because OceanPort is a subclass of City, the only candidates for OceanPort are those things we know to be cities, i.e., Paris, Zurich, & San Diego “What are the cities that connect to an ocean?” 21

22. Semantic Web for the Working OntologistDean Allemang, Jim Hendler Common Modeling Errors [9/19] - Exclusivity (Antipattern) ■ Example (cont’d) 22

23. Semantic Web for the Working OntologistDean Allemang, Jim Hendler Common Modeling Errors [10/19] - Exclusivity (Antipattern) ■ Example (cont’d) 23

24. Semantic Web for the Working OntologistDean Allemang, Jim Hendler Common Modeling Errors [11/19] - Exclusivity (Antipattern) ■ Example (cont’d) We want something to become a member of OceanPort just if it is both a City & it connects to an Ocean We do this with an intersection 24

25. Semantic Web for the Working OntologistDean Allemang, Jim Hendler Common Modeling Errors [12/19] - Exclusivity (Antipattern) ■ The Class Exclusivity fallacy is a common error for anyone who has experienced with any of a number of different modeling paradigms ■ Semantic Web modeling takes the AAA assumption more seriously than any other common modeling system 25

26. Semantic Web for the Working OntologistDean Allemang, Jim Hendler Common Modeling Errors [13/19] - Objectification (Antipattern) ■ One common source of modeling errors is attempting to build a Semantic Web model that has the same meaning & behavior as an object system ■ Object systems are not intended to work in the context of the three Semantic Web assumptions: AAA Open World Nonunique naming 26

27. Semantic Web for the Working OntologistDean Allemang, Jim Hendler Common Modeling Errors [14/19] - Objectification (Antipattern) ■ The role of a class in a model Object modeling makes little or no sense to speak of ▶ multiple classes (stamped out of two templates?) ▶ having a property that isn’t in the class (where do you put it if there wasn’t a slot in the template for it?) ■ Properties in Semantic Web models exist independently of any class can be used describe any individual at all regardless of which classes it belongs to 27

28. Semantic Web for the Working OntologistDean Allemang, Jim Hendler Common Modeling Errors [15/19] - Objectification (Antipattern) ■ Suppose an object model is intended to reflect the notion that “a person has exactly two parents who are also people” 28 RequirementsViolations 1. A value for the property hasParent can be specified only for members of the Person class AAA, Open World 2. We will recognize as a mistake the situation in which only one value for hasParent is specified for a single person Open World 3. We will recognize as a mistake the situation in which more than two values for hasParent is specified for a single person Nonunique naming

29. Semantic Web for the Working OntologistDean Allemang, Jim Hendler Common Modeling Errors [16/19] - Managing Identifiers for Classes (Antipattern) ■ In the NCI ontology, we saw a need for identifiers for classes Various entities (classes, individuals, even properties) have some sort of index number ■ The use of a property to describe a class in this way risks confusion about whether we are describing a class or an individual It seems to be an individual because it has a property 29

30. Semantic Web for the Working OntologistDean Allemang, Jim Hendler Common Modeling Errors [17/19] - Managing Identifiers for Classes (Antipattern) ■ OWL provides a simple answer to this issue A property can be declared as an AnnotationProperty, indicating that ▶ - its use in such a context has no meaning in terms of logic ▶ - thus does not make any statement about whether a subject is a class, individual, or a property 30

31. Semantic Web for the Working OntologistDean Allemang, Jim Hendler Common Modeling Errors [18/19] - Creeping Conceptualization (Antipattern) ■ Designing for reuse is enhanced by keeping things simple ■ But all too often the idea of “design for reuse” gets confused with “say everything you can” When we include ShakespeareanWork & ElizabethanWork in our model, we are tempted to further assert that ElizabethanWork is a subclass of Work, which is a subclass of IntangibleEntity And we want to include TangibleEntity, and so on 31

32. Semantic Web for the Working OntologistDean Allemang, Jim Hendler Common Modeling Errors [19/19] - Creeping Conceptualization (Antipattern) ■ You might think that modeling for reuse is best done by anticipating everything that someone might want to use your model for “The more you include the better” ■ This is a mistake The more you put in, the more you restrict someone else’s ability to extend your model ■ Reuse is best done by designing to maximize future combination with other things, not to restrict it 32