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Scenario based approaches are becoming ubiquitous in systems analysis and design but remain vague in definition and scope. A survey of current practices indicates we must offer better means for structuring, managing, and developing their use in diverse contexts. The European Esprit project Crews (Cooperative Requirements Engineering with Scenarios) are seeking a deeper understanding of scenario diversity, necessary to improve methodological and tool support for scenario based requirements engineering. They follow a two pronged strategy to gain this understanding. First, following the "3 dimensions" requirements engineering framework developed in the precursor Nature project (K. Pohl, 1994), they developed a scenario classification framework based on a comprehensive survey of scenario literature in requirements engineering, human computer interaction, and other fields. They used the framework to classify 11 prominent scenario based approaches. Secondly, to complement this research framework, they investigated scenario applications in industrial projects through site visits with scenario user projects. The article focuses on these site visits. It was found that while many companies express interest in Jacobson's use case approach, actual scenario usage often falls outside what is described in textbooks and standard methodologies. Users therefore face significant scenario management problems not yet addressed adequately in theory or practice, and are demanding solutions to these problems.

The requirements engineering, information systems and software engineering communities recently advocated scenario-based approaches which emphasise the user/system interaction perspective in developing computer systems. Use of examples, scenes, narrative descriptions of contexts, mock-ups and prototypes-all these ideas can be called scenario-based approaches, although exact definitions are not easy beyond stating that these approaches emphasise some description of the real world. Experience seems to tell us that people react to ‘real things’ and that this helps in clarifying requirements. Indeed, the widespread acceptance of prototyping in system development points to the effectiveness of scenario-based approaches. However, we have little understanding about how scenarios should be constructed, little hard evidence about their effectiveness and even less idea about why they work. The paper is an attempt to explore some of the issues underlying scenario-based approaches in requirements engineering and to propose a framework for their classification. The framework is a four-dimensional framework which advocates that a scenario-based approach can be well defined by itsform, content, purpose andlife cycle. Every dimension is itself multifaceted and a metric is associated with each facet. Motivations for developing the framework are threefold: (a) to help in understanding and clarifying existing scenario-based approaches; (b) to situate the industrial practice of scenarios; and (c) to assist researchers develop more innovative scenario-based approaches.