Spiral model

The spiral model is a software development model combining elements of both design and prototyping-in-stages, so it's a healthy mix of top-down and bottom-up concepts.

Each phase starts with a design goal (such as a user interface prototype as an early phase) and ends with the client (which may be internal) reviewing the progress thus far.

Analysis and engineering efforts are applied to each phase of the project, with an eye toward the end goal of the project.

So, for a typical shrink-wrap application, this might mean that you have a rough-cut of user elements (without the pretty graphics) as an operable application, add features in phases, and, at some point, add the final graphics.

The disadvantage of this approach is that time estimates are harder at the outset because some of the analysis isn't done until that stage is going through design. The advantage is that times get more realistic as work progresses, because the questions have been raised.

The advantage is that it is more able to cope with the (nearly inevitable) changes that software development generally entails. Another advantage is that engineers, who can get restless with protracted design processes, can get their hands in and working on a project earlier.