Concurrent accessibility of multiple actions

The most important characteristic of a GUI is that it provides the potential for giving users accessibility to many functions simultaneously. This leads to what has been characterised as multi-threaded or concurrent human-computer dialogue. As an alternative, consider the example of an automated bank teller. This is an application where users are constrained to a limited number of choices at each point in the dialogue. They may be able to choose between several services, cash, statement, new cheque book etc. but this choice is only available at a specific point in the dialogue. Such systems lead to very sequential human-computer dialogue. Concurrent accessibility is generally considered to be a good thing as it allows flexibility in the way users carry out their work and a good design principle is to maximise it, except where there are strong external constraints on the order in which tasks are to be carried out. The author outlines a method for describing and reasoning about the accessibility of functions when specifying a new product. This method encourages simultaneous accessibility to functionality