Spiral Model

Definition

The Spiral Model (proposed by Barry Boehm, 1988) is a risk-driven process model that combines iterative development (like prototyping) with the systematic, controlled aspects of the Waterfall model. Development proceeds in a spiral of repeated cycles (loops), each covering the same four quadrants.

The Four Quadrants (each loop of the spiral)

1. Determine Objectives — identify goals, alternatives, and constraints for this cycle
2. Identify & Resolve Risks — analyze risks, build prototypes if needed to reduce risk
3. Development & Testing — develop and verify the next level of the product
4. Planning — review progress and plan the next spiral cycle

                 Determine Objectives
                        ▲
        Planning                    Identify/Resolve
       for next                        Risks
       phase                          
                        ▼
              Development & Testing
        (each full loop = one spiral, radius = cumulative cost)

The spiral grows outward with each loop, representing increasing cost and completeness, starting from a small pilot and expanding toward the full system.

Key Feature: Risk Analysis

Unlike other models, the Spiral Model has a dedicated risk analysis step every cycle — making it ideal for high-risk projects.

Advantages

• Strong emphasis on risk management — reduces risk early
• Combines benefits of Waterfall (systematic) and Prototyping (iterative feedback)
• Suitable for large, complex, high-risk projects
• Allows for changes/refinement at each cycle

Disadvantages

• Complex — harder to manage than simpler models
• Requires expertise in risk analysis; risky if poor risk assessment
• Can be expensive — many iterations increase cost and time
• Not suitable for small, low-risk projects (overhead too high)

When to Use

• Large, expensive, mission-critical projects
• Projects with significant technical, cost, or schedule risk

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Exam tip: Always draw the 4-quadrant spiral diagram — this is one of the most commonly drawn diagrams in SE exams. Mention Boehm (1988) as originator.