13. Waterfall model

With the waterfall method, if everything goes perfectly, then the project 'cascades' through the System Life Cycle stages, one after another.

A variation of the waterfall model allows for the fact that if something changes, such as the user requirements then part of the the project has to go back to an earlier stage.

The Waterfall method is good for well understood and stable user requirements such as upgrading a well-used existing system.

It is not so good for a brand new system with a large number of unknowns or uncertainties. In that case the spiral method described on the next page is considered.