Alistair's article of adaptive sampling make a good point about quality not needing to be uniform:
"... First of all requirements statements aren't requirements, they are only placeholder tokens for [need-solution] pairs. Secondly, the system doesn't have a single quality requirement, the quality characteristics vary by [need-solution] pairs. We can save time and money on the project by examing each "requirement" (or wish, or user story, or backlog item) individually, and working out what the appropriate level of effort will be needed at that one spot. We can save time and money in that endeavor by using [this] adaptive sampling idea and breaking the work into areas, those with similar quality needs..."