Prototypes are the shell of an actual production application and are used to provide insight into the look, feel and flow of an application. The main purpose is to gather requirements related to the user interface. A “throw away” prototype seeks to quickly uncover and clarify system interface requirements. It is especially useful in cases where the most efficient workflow can only be discovered with hands-on experience.

Some have made the mistake of thinking prototyping is all you need to do to gather requirements. This will almost always be a costly mistake! Other requirements techniques will need to be used to get a complete set.

When you obtain the user interface requirements using prototyping, the next step is to integrate these requirements with your use cases, scenarios, data model and business rules. Your new discoveries will usually result in changes to previously gathered requirements.

There are two categories of prototypes used to uncover functional scope:

1. Horizontal Prototype – models a shallow and wide view of the system’s functionality. It typically does not have any business logic running behind the visualization.
2. Vertical Prototype – models a deep and narrow slice of the system’s functionality. There usually will be some business logic built to uncover complex requirements.

It is a judgment call as to whether or not to use prototyping on a particular project. You must weigh factors such as the importance of the user interface, the clarity of the requirements gathered via other methods, the difficulty in building the prototype, and time constraints.