Our analyses of the activities performed by users of computer systems show complex patterns of interleaved activities. Current human - computer interfaces provide little support for the kinds of problems users encounter when attempting to accomplish several different tasks in a single session. In this paper we develop a framework for discussing the characteristics of activities, in terms of activity structures, and provide a number of conceptual guidelines for developing an interface which supports activity coordination. The concept of a workspace is introduced as a unifying construct for reducing the mental workload when switching tasks, and for supporting contextually-driven interpretations of the users' activity structures.

This paper outlines a set of proposals for the development of system documentation based on an analysis of user needs. It is suggested that existing documentation is not sensitive enough to the variety of levels of user expertise, nor to the variety of contexts in which on-line help is required. We outline three specific proposals for fulfilling these needs: a quick reference facility, a command-line database, and a facility for full explanation and instruction, and suggest a number of ways in which users might access these facilities. Finally, we suggest a way of combining these facilities into an integrated structured manual, offering more effective user support than is currently provided.