Use lake a full understanding of their needs, or their needs change rapidly.
System requirements are hard to define.
System input and output are not known.
The task to be performed is unstructured or semi structured.
Designers are uncertain about what technology to use.
The system to be developed is crucial and needed quickly.
The risk associated with developing the wrong system is high.
The user's reactions to the new system are important development considerations.
Many design strategies must be tested.
The design staff has litter experience developing the system or application under consideration.
The system will be used infrequently (and therefore processing efficiency is not a major concern.)