User feedback from initial and later testing is a valuable source of require- ments information. Interviews, discussions, and model reviews are an imperfect way of eliciting complete and accurate requirements. The phrase"I'll know it when I see it applies well to requirements definition. Users often cannot completely or accurately state their requirements until they can interact with a live system that implements those requirements. Based on those interactions, users can develop concrete suggestions for improvement and identify missing or requirements. poorly implemented