Advantages of the Spiral Model
The key is continual development; it is intended to help manage risks. You should not define the entire system in detail at first. The developers should only define the highest- priority features. This type of development relies on developing prototypes and then giving them back to the user for trial. With this feedback the next prototype is created. Define and implement this, then get feedback from users / customers (such feedback distinguishes 'evolutionary' from 'incremental' development). With this knowledge, you should then go back to define and implement more features in smaller chunks, until an acceptable system is delivered.
The advantages of using the spiral model are varied: its design flexibility allows changes to be implemented at several stages of the project; the process of building up large systems in small segments makes it easier to do cost calculations; and the client, who will be involved in the development of each segment, retains control over the direction and implementation of the project. In addition, the client's knowledge of the project grows as the project grows, so that they can interface effectively with management.
The Rapid Application Development methodology was developed to respond to the need to deliver systems very fast. The RAD approach is not appropriate to all projects - an air traffic control system based on RAD would not instill much confidence. Project scope, size and circumstances all determine the success of a RAD approach. The following categories indicate suitability for a RAD approach.
Next: Suitability of RAD