Software development often proves far more expensive than expected. Evidence indicates that the earlier a defect is discovered in development, the less impact it has on both the timescales and cost. Bugs discovered late in the development cycle send costs soaring and risk the integrity and safety of a system, especially if the software has been deployed. Obviously, careful planning, organization, and a team with the correct skills all help.
Since its inception in the early 1970s, the sequential waterfall model has served as a framework for software development alternatives. In this model, each phase cascades to the next, which only starts when the defined goals for the previous phase are achieved.
In practice, earlier phases often need to be revisited as developers work iteratively and requirements come together as users test prototype versions of the system. Because of this iterative approach, it is even more important to apply suitable verification and validation (V&V) techniques at each stage and within each iteration.