A spiral development model is a risk-based model that introduces functionality in incremental steps, allowing competing designs to be explored while the product is in development. Each cycle of the spiral begins with a determination of the objectives, alternatives, and constraints. It is followed by risk analysis leading to a prototype. The prototype(s) are verified and validated and the cycle is concluded with a review of the progress made in this cycle. Then the cycle begins again and with each successive cycle, the design is refined and full functionality is approached. Benefits of this approach include allowing competing technologies to be compared in the context of the system, as well as allowing for a more rapid refinement of a product. The development model focuses on minimizing the program's risk. The complexity of systems, market pressures, other schedule considerations, and cost constraints have driven many system developers to use a spiral development model in the development of systems and software. This development model presents interesting problems as well as opportunities for the system safety engineer. This article explores these benefits and considerations, and recommends an approach to manage the software safety effort over the product life.
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