in Usability Requirements Engineering
GMD German National Research Center for Information Technology
The major purpose of requirements engineering is to achieve a common understanding between the user and the designer of an intended software product. Scenarios describe the user's view of performing tasks with a software product in terms of the user's language ( Carroll 1994, 1995). Prototypes represent the designer's ideas initially realized as an arrangement of functions at the envisaged user interface. Both kind of representations, scenarios and prototypes, form a symbiosis in the design process ( Weidenhaupt et al., 1998). Scenarios serve as a catalyst when designing and validating prototypes. Prototypes are in response to scenarios and serve as stimuli to users for describing more elaborated scenarios. This paper illustrates how the use of scenarios can guide the prototyping process by directing the designer's focus on task and context-related issues when arguing about technical features concerning the user interface. The role of two types of scenarios is considered: context and use scenarios. (For the usage of scenarios in usability requirements engineering see also the paper of Wolfgang Dzida in this volume).
Prototyping has been accepted as a method to involve the user early in the development process of a software product. A prototype is intended to give the prospective user a chance to argue about the fitness of tangible design proposals to the required results of user task performance. In usability engineering prototyping serves two purposes: first, prototyping as a process is challenging the designer to demonstrate an elaborated understanding of the user requirements by transforming them into product attributes and second, the prototype as a design proposal is enabling the user to check whether a consensus about the suggested attributes can be achieved. Furthermore, the User is encouraged to reflect his requirements in view of the potential of a technology and concrete technical alternatives. This kind of prototyping is also refered to as exploratory.
The idea of exploratory prototyping is to demonstrate and examine alternative design options ( Budde et al., 1992) in order to come to an initial design proposal. Discussing various ways of performing a task in view of a prototype reveals trade-offs and misunderstandings but leads also to more concrete, wellfounded requirements on the user's side ( Ryan and Doubleday, 1997). Alternative design options can be submitted to claims analysis ( Carroll and