Boehm's Spiral Model

This process model is characterised by the recognition of risk factors within software system production. Risks, being a consequence of inadequate information, are resolved by gathering information to reduce the level of uncertainty. There are no fixed phases, however each phase is divided into four stages. Each round of the spiral is equivalent to one phase of the development process. The Waterfall and Evolutionary models are subsets of the spiral model.

• Objectives - The aims of the analysis.
  
• Constraints - Limiting factors.
  
• Alternatives - Other methods of achieving the objectives.
  
• Risks - The possible risks with identified alternatives.
  
• Risk resolution - Strategies to reduce identified risks.
  
• Results - The outcomes of the risk resolution strategies.
  
• Plans - How to tackle the next phase.
  
• Commitment - Management decisions on how to continue.

The quadrants into which each phase is divided structure the process as follows :

1. Objective setting
   The specific objectives of this phase of the project are defined. Constraints on the process and product are identified and a detailed management plan drawn up. Project risks are identified. Alternative strategies, depending on these risks, may be planned.
2. Risk assessment and reduction
   For each identified risk, a detailed analysis is carried out. Steps are taken to reduce the risk.
3. Development and validation
   One of the standard development models for the system is chosen. This may be evolutionary prototyping, the waterfall model or formal transformations.
4. Planning
   The project is reviewed and a decision made whether to continue with a further loop of the spiral. If it is decided to continue, plans are drawn up for the next phase of the project.

Disadvantages
Too involved for small scale projects.

Advantages
Reduces risk factors within the development process. Excellent process visibility with documentation produced at each quadrant and phase.