“An essential use case is a structured narrative, expressed in
the language of the application domain and of users,
comprising a simplified, generalized, abstract, technology-free
and implementation independent description of one
task or interaction that is complete, meaningful, and well-defined
from the point of view of users in some role or roles
in relation to a system and that embodies the purpose or
intentions underlying the interaction” (Constantine and Lockwood (1999).
Said another way, an essential use case describes the interaction between the user and the system at a high level of abstraction. The goal of an essential use case is to convey the most important aspects of the user-system interaction by focusing on the user’s intent (avoiding any reference to an assumed UI design or technological implementation) and on the observable result of the system (without specifying the internal steps the system takes to achieve the result). Since an essential use case describes only the most important information it represents a single success scenario.
In contrast, a system use case describes the interaction between the user and system in a more detailed way than and essential use case. While still trying to avoid referencing any UI specific features when possible, usually certain aspects of the technology to be used can be assumed. For instance, when writing a system use case, it is usually known whether the user will interact with a telephonic system, an internet application, or a piece of manufacturing equipment. Similarly, system use cases provide more detailed description of the steps that the system will perform to fulfill the need of the user. In order to avoid committing to a specific UI design, this detail should still be expressed in logical terms. However, it paints a clearer picture of the requirements that the GUI must satisfy.
References:
Constantine, Larry L., and Lucy A. D. Lockwood. 1999. Software for Use: A Practical Guide to the Models and Methods of Usage-Centered Design. Addison-Wesley Professional. |