Analogue representations are usually best for the design of interfaces in perceptual-motor tasks such as car driving. Many computer-based tasks, though, are carried out by manipulating symbols rather than performing skills. Skilled performance is graded - we can talk about the performance of task elements (such as operating the controls, performing hill starts and reacting to traffic signals) in terms of the degree of success or safety with which they were carried out. Performance can be assessed and scored on a continuum or scale. Symbol-based human-computer interaction is much more brittle. Symbols always have a certain arbitrariness about them that increases the requirement for learning and the likelihood of forgetting. When recognising a symbol, we are usually either right or wrong - there are no shades of grey. Much of the effort in human-computer interaction (HCI) research in the last 20 years has been directed at making interactive systems easier to use. To master any interactive device, users have to learn about
People forget ideas not consonant with their predispositions but retain without loss or even increment those ideas consonant with their predispositions.
(Hovland et al., 1949)
|• The characteristics of the device|
|• The tasks be be performed with the device|
|• How to perform the tasks using the device|
|• Making a new task similar to one that is already known|
|• Striving for conceptual and operational consistency with known tasks|
A number of frameworks have been posited to guide designers through the issues associated with designing user-friendly interactive systems. In Command Language Grammar (Moran, 1981), a computer system is represented at several different levels, as described below.
At the conceptual level, we distinguish between the user's mental model of the system, the designer's mental model and the real system. Designers should strive to ensure that the design model is compatible with the real system (its essential features