7.1 Usability Specifications for Evaluation

Mediated evaluation is a key idea behind usability specifications (Carroll & Rosson 1985; Whiteside, Bennett, & Holtzblatt 1988), which are precise and testable statements of the usability characteristics required of a system. The intention is to specify and manage usability goals in parallel with the functional goals for the system. Thus, at every step in the development process, as system functions are designed and specified, their consequences for users and their activities are also analyzed and specified. This parallel specification process ensures that usability concerns will always be considered and assessed as new features are proposed and incorporated into the design. In scenario-based development, the usability specifications are derived directly from design scenarios.

Usability specifications rely on a task analysis similar to the methods of hierarchical task analysis described in Chapter 2. A user interaction scenario describes work activities that are typical, critical, or novel. Task analysis breaks these activities into subtasks that provide a more precise specification of what users are expected to do. For example, a scenario in which a user "searches a digital library for Scriven's article on evaluation" might be analyzed into subtasks such as "accessing the digital library," "initiating the search function," and "specifying search keys for Scriven's article." Decomposing a scenario in this way allows the evaluator to state exactly what a given user should be able to do, and what level of performance and/or satisfaction is expected for each subtask (Table 7.2). The DEC project described in...