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6.4 Lifecycle models: showing how the activities relate 189

Cumulative

through
steps

Review
----___

Plan next phases

Develop, verify
next-level product
Figure 6.9 The spiral lifecycle model of software development.

hind RAD began to emerge in the early 1990s, also in response to the inappropri-
ate nature of the linear lifecycle models based on the waterfall. Two key features of
a RAD project are:
Time-limited cycles of approximately six months, at the end of which a sys-
tem or partial system must be delivered. This is called time-boxing. In effect,
this breaks down a large project into many smaller projects that can deliver
products incrementally, and enhances flexibility in terms of the development
techniques used and the maintainability of the final system.