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16 Chapter 1 What is interaction design?

confirmatory dialog boxes that give users another chance to consider their inten-
tions (a well-known example used in e-mail applications is the appearance of a dia-
log box, after the user has highlighted messages to be deleted, saying: "Are you
sure you want to delete all these messages?" See Figure 1.6(b)).
Question: Does the system prevent users from making serious errors and, if
they do make an error, does it permit them to recover easily?
Utility refers to the extent to which the system provides the right kind of func-
tionality so that users can do what they need or want to do. An example of a system
with high utility is an accounting software package providing a powerful computa-
tional tool that accountants can use to work out tax returns. A example of a system
with low utility is a software drawing tool that does not allow users to draw free-
hand but forces them to use a mouse to create their drawings, using only polygon
shapes.
Question: Does the system provide an appropriate set of functions that enable
users to carry out all their tasks in the way they want to do them?
Learnability refers to how easy a system is to learn to use. It is well known that
people don't like spending a long time learning how to use a system. They want to
get started straight away and become competent at carrying out tasks without too
much effort. This is especially so for interactive products intended for everyday use
(e.g., interactive TV, email) and those used only infrequently (e.g., videoconferenc-
ing). To a certain extent, people are prepared to spend longer learning more com-
plex systems that provide a wider range of functionality (e.g., web authoring tools,
word processors). In these situations, CD-ROM and online tutorials can help by
providing interactive step-by-step material with hands-on exercises. However,
many people find these tedious and often difficult to relate to the tasks they want to