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3 Types of Requirement

Priority-driven processes only work if you can model the process around task patterns for a single requirement. There is little point in prioritizing say, coding over testing, or design over specification, since all these activities must take place at some stage, even if they happen inside an activity that is called something else. But are all requirements the same (in the sense of using the same processes) or are some requirements more the same than others?!

Recently I was asked to advise on the requirements capture process for an organization wanting to apply agile techniques in a controlled environment. I came up with some slightly different names for types of requirement, though I think the concepts will be familiar to you if you've analyzed other software development methods. The requirements types (shortened to PCF) are as follows:
  • Business problem statements (Problems)
  • Solution contraint statements (Constraints)
  • Solution feature statements (Features)
I've written elsewhere about FDD, which provides a very good process for managing and prioritizing features. However often projects applying processes like FDD do not pay enough attention to the other types of requirement that precede features.

Problems, for example, should identify issues in terms of a measurable aspect of the current solution and the degree to which a performance improvement would overcome it. Constraints express aspects of the solution that the current designers are not expected to change (though they should also state the rationale behind the constraint, and how and by whom it may be changed if justification exists). Only by considering all of these three types of requirement can the essential requirements of a system be captured.
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