A software life-cycle model devised by Barry Boehm that encompasses a management strategy, a life-cycle development model, and risk assessment. The model takes its name from the spiral representation. Beginning at the centre of the spiral, where there is limited detailed knowledge of requirements and small costs, successive refinement of the software is shown as a traverse around the spiral, with a corresponding accumulation of costs as the length of the spiral increases. Interaction between phases is not shown directly since the model assumes a sequence of successive refinements coupled to decisions that project risks associated with the next refinement are acceptable. Each 360° rotation around the centre of the spiral passes through four stages: planning, seeking alternatives, evaluation of alternatives and risks, and (in the lower right quadrant) activities equivalent to the phases represented in waterfall or V-models.