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188 Chapter 6 The process of interaction design

design can begin. The names given to these steps varies, as does the precise defi-
nition of each one, but basically, the lifecycle starts with some requirements
analysis, moves into design, then coding, then implementation, testing, and fi-
nally maintenance. One of the main flaws with this approach is that require-
ments change over time, as businesses and the environment in which they
operate change rapidly. This means that it does not make sense to freeze re-
quirements for months, or maybe years, while the design and implementation
are completed.
Some feedback to earlier stages was acknowledged as desirable and indeed
practical soon after this lifecycle became widely used (Figure 6.8 does show some
limited feedback between phases). But the idea of iteration was not embedded in
the waterfall's philosophy. Some level of iteration is now incorporated in most ver-
sions of the waterfall, and review sessions among developers are commonplace.
However, the opportunity to review and evaluate with users was not built into this
model.

The spiral lifecycle model
For many years, the waterfall formed the basis of most software developments, but
in 1988 Barry Boehm (1988) suggested the spiral model of software development
(see Figure 6.9). Two features of the spiral model are immediately clear from Fig-
ure 6.9: risk analysis and prototyping. The spiral model incorporates them in an it-
erative framework that allows ideas and progress to be repeatedly checked and
evaluated. Each iteration around the spiral may be based on a different lifecycle
model and may have different activities.
In the spiral's case, it was not the need for user involvement that inspired the
introduction of iteration but the need to identify and control risks. In Boehm's ap-
proach, development plans and specifications that are focused on the risks involved
in developing the system drive development rather than the intended functionality,
as was the case with the waterfall. Unlike the waterfall, the spiral explicitly encour-
ages alternatives to be considered, and steps in which problems or potential prob-
lems are encountered to be re-addressed.
The spiral idea has been used by others for interactive devices (see Box 6.4). A
more recent version of the spiral, called the WinWin spiral model (Boehm et al.,
1998), explicitly incorporates the identification of key stakeholders and their re-
spective "win" conditions, i.e., what will be regarded as a satisfactory outcome for
each stakeholder group. A period of stakeholder negotiation to ensure a "win-win"
result is included.

Rapid Applications Development (RAD)
During the 1990s the drive to focus upon users became stronger and resulted in a
number of new approaches to development. The Rapid Applications Development
(RAD) approach attempts to take a user-centered view and to minimize the risk
caused by requirements changing during the course of the project. The ideas be-