Software Engineering for Students A Programming Approach

5.5 Design principles and guidelines 57
A novice user or an occasional user is not likely to remember much about how to
use the system. Thus a direct manipulation interface may be the most suitable
approach. But an experienced and frequent user may be frustrated by an interface
designed for novices and may prefer shortcut commands and/or a command line
interface. For example, a number of applications provide a macro facility, in which a
series of commands can be grouped together, parameterized and invoked as a single
command. Again the need for flexibility in the interface becomes apparent.
The skill level of the end user has a significant impact on the ability to extract meaningful
information from the user interface, respond efficiently to tasks that are demanded
by the interaction, and effectively apply heuristics that create a rhythm of interaction.
It seems that context- or domain-specific knowledge is more important than overall
education or intelligence. For example, an engineer who uses a computer-based diagnostic
system to find faults in automobiles understands the problem domain and can
interact effectively through an interface specifically designed to accommodate users
with an engineer’s background. This same interface might confuse a physician, even
though the physician has considerable experience of using a computer for diagnosing
illnesses in patients.
The software developer may unconsciously incorporate into the user interface some
assumptions about the implementation that are irrelevant or even confusing for the
users. Consider a word processor, for example. What the user wants is to create and edit
documents, and they know that documents reside in files on a disk. The user probably
understands the concept of opening a file, because this is a familiar concept in using
manual files, but the idea of saving a file may well be completely mysterious to them.
The reason is that the concept of saving a file derives from the developer’s mental model
of how a word processor works, that is, it keeps all or part of the document in main
memory. This example illustrates how the designer can get it wrong and therefore the
importance of the involvement of the user in design.
In conclusion, there are a number of different viewpoints taken by the users and
developers of a user interface. There is scope for either conflict or harmony between
these views. Conflict between the users’ perception and the developers’ concepts can
make for a system that is difficult to use, but involving the users in the design can
assist in recognizing users’ views, and flexibility in the interface can help cater for different
users.
5.5 ● Design principles and guidelines
Design principles are high-level principles that can guide the design of a user interface.
Three overall principles are:
■ learnability – how easily can new users learn to use the system?
■ flexibility – does the interface support a variety of interaction styles? (We have
already seen why this is an important consideration.)
■ robustness – how much feedback does the system give the user to confirm what is
going on?