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Should Optional Properties Be Used in Conceptual Modelling? A Theory and Three Empirical Tests

Institut d'Informatique, Facultés Universitaires Notre-Dame de la Paix, rue Grandgagnage, 21, B-5000 Namur, Belgium
Department of Accounting and Financial Management, The University of the South Pacific, Suva, Fiji
Department of Commerce, The University of Queensland, Australia 4072
Department of Commerce, The University of Queensland, Australia 4072
fbodart{at}info.fundp.ac.be
patel_a{at}usp.ac.fj
sim{at}commerce.uq.edu.au
weber{at}commerce.uq.edu.au

An important feature of some conceptual modelling grammars is the features they provide to allow database designers to show real-world things may or may not possess a particular attribute or relationship. In the entity-relationship model, for example, the fact that a thing may not possess an attribute can be represented by using a special symbol to indicate that the attribute is optional. Similarly, the fact that a thing may or may not be involved in a relationship can be represented by showing the minimum cardinality of the relationship as zero. Whether these practices should be followed, however, is a contentious issue. An alternative approach is to eliminate optional attributes and relationships from conceptual schema diagrams by using subtypes that have only mandatory attributes and relationships.

In this paper, we first present a theory that led us to predict that optional attributes and relationships should be used in conceptual schema diagrams only when users of the diagrams require a surface-level understanding of the domain being represented by the diagrams. When users require a deep-level understanding, however, optional attributes and relationships should not be used because they undermine users' abilities to grasp important domain semantics. We describe three experiments which we then undertook to test our predictions. The results of the experiments support our predictions.

Key Words: Database Design; Data Models; Entity-Relationship Model; Semantic Data Models; Systems Theory; Ontology; Optional Attributes; Optional Relationships; Subtyping
History: This paper was received on September 30, 1999.

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