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Fig. 2.3 A summary of the spiral development model (adapted from Boehm 1988 )
Invariant 1 is the concurrent definition of key artifacts, such as concept,
requirements, plan, design, and coding. It is argued that de
ning these artifacts in a
sequential structure can constraint the project to excessively rigid preconceptions.
Invariant 2 is that each cycle follows the four strategic principles that corre-
spond to the four quadrants of the model: determine objectives, evaluate risks,
develop and test, and plan the next iteration. Not moving in accordance with this
basic strategy can negatively affect the entire process.
Invariant 3 is that the level of effort is determined by the risk considerations.
Reasonable time frames must be established for each project in accordance with risk
assessments, to determine
in each activity.
Invariant 4 is that the degree of detail is driven by risk considerations. Just like
invariant 3, here it is important to determine
how much is enough
how much detail is enough
in each
stage of the process.
Invariant 5 refers to the use of anchor point milestones, which Boehm describes
as
Life Cycle Objectives (LCO), Life Cycle Architecture (LCA) and Initial
Operational Capability (IOC)
(Boehm 2000 ). At each of the anchor points,
stakeholders will review the key artifacts of the stage.
Finally, invariant 6 states that besides the construction aspects, the development
process needs to focus also on the overall life cycle itself. This means that long-term
concerns should always be taken into account.
 
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