Decision Matrix: Waterfall vs Incremental vs Spiral for a Safety-Critical System

Selection criteria

For a safety-critical system a failure can cause injury or loss of life, so risk handling matters most. The other criteria are requirement volatility, feedback and delivery.

Comparative matrix

Criterion	Waterfall	Incremental	Spiral
Risk	Weak: risks surface late, mostly during testing	Moderate: risk handled per increment	Strong: explicit risk analysis in every loop
Requirement volatility	Poor: requirements frozen early	Good: new needs go into later increments	Good: requirements refined every loop
Feedback	Late: only after testing/delivery	After every increment	After every loop (prototype + review)
Delivery	Single big-bang release	Early partial working releases	Versions/prototypes; final release after several loops

Weighted score (scale 1 to 5, illustrative)

Criterion (weight)	Waterfall	Incremental	Spiral
Risk (0.4)	2	3	5
Volatility (0.2)	1	4	4
Feedback (0.2)	1	4	4
Delivery (0.2)	2	5	3
Weighted total	1.6	3.8	4.2

Example: Spiral = 0.4(5) + 0.2(4) + 0.2(4) + 0.2(3) = 4.2.

Justification

• Risk has the highest weight because hazards in a safety-critical system must be found and removed early.
• Spiral scores highest: each loop has a risk-analysis step, prototypes are evaluated, and verification can be repeated before the next loop.
• Waterfall gives strong documentation but finds risks too late and cannot absorb change.
• Incremental gives early feedback but has no built-in risk analysis.

Decision: Spiral, keeping Waterfall-style rigorous documentation, reviews and verification inside each loop. If the requirements are well understood and risk is low, Incremental is the fallback.

Note: scores and weights are judgement-based; state your assumptions in the exam.