Spiral Model

This is one of the activity based iterative models. It deals with iterations and changes in activities. All of the waterfall's activities are extended into a cycle. Each cycle consists of four phases. In the first phase determination of objectives, alternatives and constraints happens. Alternatives are evaluated as well as risks being identified and resolved in the second phase. In the third phase the development and verification of the current cycle happens. The last phase is used to plan the next cycle. For each cycle the distance from the center measures its proximity to the final product and cost whereas its angular coordinates show the overall progress.

An advantage of this model is its ability to connect objectives to product developments. Risks management is also considered as well as iterations of tasks. It actually combines both development and evolutionary approaches. But the most important advantage is that there are prototypes even if the whole project has to be cancelled.

Nevertheless, it's quite hard to apply in real world scenarios. This model demands many activities in each cycle and requires quite some knowledge in risk management to consider all of the necessary circumstances. Changes in between the cycles are possible but not within one cycle.