Describing the UserThe Users' Domain

The entities in our model of the user depict referents in the user's world. These referents often correspond to everyday physical objects. Equally often referents correspond to concepts that—though not physical objects—take on concrete reality when the user manipulates these referents as a part of their tasks.

Operational Definitions

The first and easiest way to define a set of entities is to provide an operational definition of each entity. The operational definition provides a terse description of the entity much like a dictionary definition would. In fact, one very useful application of these operational definitions is in creating a user-domain glossary.

The first use of these definitions is to drive towards consistency in meaning across the definitions and consistency of usage between entities. When a reasonable first-pass at these definitions is available they are used to derive more formal descriptions of the entities.

Formalizing Entity Descriptions

Operational definitions, as useful as they are, are informal and do not capture all of the regularity in domain description that the usability professional can later take advantage of. These regularities include describing the relationships among a set of entities and describing the attributes of each entity.

The reason operational definitions are useful is that these short entity descriptions capture the essence of an entity's relationship to other entities. It becomes easier for the usability professional to compare these descriptions and provide a more consist account of the relationship of one entity to another. But the difficulty with operational definitions is that this description of inter-relationship is compartmentalized into separate definitions forcing the modeler to build up a mental image of the domain as a whole. Formal modeling techniques provide a structured method of creating a picture of related entities including an explicit description of the nature of the relationships among them.

Relationships

Cognitive psychologists have described a number of regularities in the way people think about relationships among objects. These include relationships that identify:

• Categories of objects and category hierarchies (is_a relationships)
• Kind-Of relationships
• Whole-part relationships
• Semantic relatedness also refered to as associations—objects that work together, but more loosely than whole-part

These relationships extract from the entity descriptions the parts of the descriptions that mention other entities. This has two important consequences for the modeler. One consequence is that unlike operational definitions, relationships explicitly avoid the authoring problem of duplicating description content in multiple places. Rather than describing the relationship as part of the entity description at both ends, the relationship becomes its own entity with its own description. The second consequence of reifying relationships is that diagrams like the one below can depict groups of entities and relationships and the larger meaning that they collectively construct. This bigger picture can be very helpful in speeding up understanding of a user's domain.

Attributes

An attribute is a part of an Entity that has no real meaning on its own and which is extremely general in nature and very simple. Typical examples are names, heights, descriptions, and colors. These can all be represented by text or numbers or fixed lists of names as with lists of colors.

In object modeling attributes are usually thought of as holders of some value that has some predefined meaning for an object. For example, a height attribute contains a number that is interpreted as a height.

In user modeling, it is useful to describe special types of attributes representing the kinds of attributes for which we have user interface display mechanisms.

The figure below shows a UML class diagram depicting a user referent as an entity, the attributes of that entity, and that entity's relationships to other entities.

Figure: a UML class diagram, over 650 pixels wide—click here to open it in a separate window.

This diagram shows how each entity is defined by the attributes it contains and the relationships to other entities that begin and end with it.

By explicitly capturing each type of relationship the modeler can base UI design patterns on the distinctions these relationships embody.