A method for assessing the usability of graphical, direct‐manipulation style interfaces

A model‐based method for assessing the usability of graphical, direct‐manipulation style interfaces was developed. The method involves collecting and integrating verbal protocol data, history logs, and videotapes of the system display. Then, an analyst familiar with the task, the data, and Norman's (1986) user activity model reviews the data and makes determinations on what they mean in terms of the model. An encoding scheme is next applied to the integrated data, to structure the Human Computer Interaction (HCI) process at a detailed interaction level. The structured data now support the application of quantitative methods and the identification of meaningful patterns and frequencies that highlight potential usability problems or instances of indirectness. Error encodings reflect user‐system interface difficulties not only in the execution stage but also in the psychological stages. The method was used to evaluate the usability of a military airspace scheduling system; the types of usability problems identified and the advantages of the method are discussed.