Editing Spiral model (section)

===Define artifacts concurrently===
Sequentially defining the key artifacts for a project often increases the possibility of developing a system that meets stakeholder "win conditions" (objectives and constraints).

This invariant excludes “hazardous spiral look-alike” processes that use a sequence of incremental waterfall passes in settings where the underlying assumptions of the waterfall model do not apply. Boehm lists these assumptions as follows:
# The requirements are known in advance of implementation.
# The requirements have no unresolved, high-risk implications, such as risks due to cost, schedule, performance, safety, user interfaces, organizational impacts, etc.
# The nature of the requirements will not change very much during development or evolution.
# The requirements are compatible with all the key system stakeholders’ expectations, including users, customer, developers, maintainers, and investors.
# The right architecture for implementing the requirements is well understood.
# There is enough calendar time to proceed sequentially.
In situations where these assumptions do apply, it is a project risk not to specify the requirements and proceed sequentially. The waterfall model thus becomes a risk-driven special case of the spiral model.
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