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Numbers Game

Use cases (see also Chapters 22 and 46), which serve many purposes in object-oriented software engineering, can also be used as the basis for some powerful and practical user interface design metrics. Three such metrics—Essential Efficiency, Task Consolidation, and Task Concordance—have been devised over several years as part of a larger suite to help designers in evaluating and improving their designs. These metrics have been crafted to have a sound and straightforward basis in established principles of software usability, such as simplicity, efficient operation, visibility of features, and meaningful organization.

Use cases define the external functionality a system must supply, and an essential use case embodies the bare-bones, idealized essentials of some well-defined task that a user would want to accomplish (see Chapter 46). The narrative body for an essential use case thus describes user-system interaction reduced to its simplest, most abstract and generalized form, devoid of assumptions about or references to specific user interface components or technology. It describes an ideal in terms of the minimal interaction necessary to accomplish a given task. Any actual user interface design can be expected to take more discrete steps to complete than in the ideal interaction. The essential use case narrative thus represents a target against which to gauge the quality of any practical design.


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