The keystroke-level model for user performance time with interactive systems
There are several aspects of user-computer performance that system designers should systematically consider. The authors propose a simple model, the keystroke-level model, for predicting one aspect of performance: the time it takes an expert user to perform a given task on a given computer system. The model is based on counting keystrokes and other low-level operations, including the user's mental preparations and the system's responses. Performance is coded in terms of these operations and operator times summed to give predictions. Heuristic rules are given for predicting where mental preparations occur. When tested against data on 10 different systems, the model's prediction error is 21 percent for individual tasks. An example is given to illustrate how the model can be used to produce parametric predictions and how sensitivity analysis can be used to redeem conclusions in the face of uncertain assumptions. Finally, the model is compared to several simpler versions. The potential role for the keystroke-level model in system design is discussed.
Card, S. K. ; Moran, T. P. ; Newell, A. The keystroke-level model for user performance time with interactive systems. Communications of the ACM. 1980 July; 23 (7): 396-410.