**5.4 Spiral model**

The spiral model is an amalgamation of the ideas of an iterative model and waterfall model. In this model, the team addresses the requirements with incremental releases of the software through iterations around the spiral effectives decreases pending requires to be addresses in future pending spiral iterations. Instead of predefined set of requirements for each iteration and development stages, the spiral model is designed to empower development teams to select the most appropriate approach given the current goals, constraints, and risks [9]. Furthermore, the number of spiral iterations and requirements addressed in each iteration are not fixed and hence, the model allows additional flexibility to the development teams for take the most appropriate course of actions based on situation. Decreasing number of pending requirements with each iteration creates a spiral effect. Spiral model-based development can be effective in a variety of situations such as varying level of risks, project length, scope changes, as well as financial resources.
