Schemes for activity reuse are based upon the assumption that the human–computer dialog has many recurring activities. Yet there is almost no empirical evidence confirming the existence of these recurrences or suggestions of how observed patterns of recurrences in one dialog would generalize to other dialogs. The next few chapters address this dearth. They provide empirical evidence that people not only repeat their activities, but that they do so in quite regular ways. This chapter starts with the general notion of recurrent systems, where most users predominantly repeat their previous activities. Such systems suggest potential for activity reuse because there is opportunity to give preferential treatment to the large number of repeated actions. A few suspected recurrent systems from both non-computer and computer domains are examined in this context to help pinpoint salient features. Particular attention is paid to repetition of activities in telephone use, information retrieval in technical manuals, and command lines in UNIX. The following chapters further examine UNIX as a recurrent system, and then generalize the results obtained into a set of design properties.

A definition of recurrent systems

An activity is loosely defined as the formulation and execution of one or more actions whose result is expected to gratify the user's immediate intention. It is the unit entered into incremental interaction systems (as defined in Section 1.2.1) (Thimbleby, 1990). Entering command lines, querying databases, and locating and selecting items in a menu hierarchy are some examples.