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Spiral Methodology:

While the waterfall methodology offers an orderly structure for software
development, demands for reduced time-to-market make its series steps inappropriate. The
next evolutionary step from the waterfall is where the various steps are staged for
multiple deliveries or handoffs.  The ultimate evolution from the water fall is the
spiral, taking advantage of the fact that development projects work best when they are
both incremental and iterative, where the team is able to start small and benefit from
enlightened trial and error along the way. 

The spiral methodology reflects the relationship of tasks with rapid
prototyping, increased parallelism, and concurrency in design and build activities. The
spiral method should still be planned methodically, with tasks and deliverables identified
for each step in the spiral.

Waterfall Methodology

All projects can be managed better when segmented into a hierarchy of
chunks such as phases, stages, activities, tasks and
steps.  In system development projects, the simplist rendition of this is called the
"waterfall" methodology, as shown in the following figure:

In looking at this graphic, which was for major defense systems
developments, please note this presumes that the system requirement have already been
defined and scrubbed exhaustively, which is probably the most important step towards
project success.  Nevertheless, the graphic illustrates a few critical principles of
a good methodology:

• Work is done in stages,


• Content reviews are conducted between stages, and


• Reviews represent quality gates and decision points for continuing.

The waterfall provides an orderly sequence of development
steps and helps ensure the adequacy of documentation and design reviews to ensure the
quality, reliability, and maintainability of the developed software.  While
almost everyone these days disparages the "waterfall methodology" as being
needlessly slow and cumbersome, it does illustrate a few sound principles of life cycle
development.