A system cannot be completely specified without feedback loops. Some DFSS techniques can be used during the Requirements & Design stages and some Agile techniques should also be used.

The trick is to know when to proceed from completing the specification to investigating the details necessary to start the design. Similarly for proceeding to implementing the design. Most people rely on an IEEE 830 specification standard which leads to a printed document that must be signed before design can start. This is not a reasonable approach. A requirements tracking tool should be used to track the requirements through their lifecycle with the necessary signatures. Instead of approving all requirements, each requirement is approved electronically allowing work to continue on approved requirements while other requirements are being discussed.

“Peter Wegner at Brown University demonstrated that it is not possible to fully specify or test an interactive system designed to respond to external inputs, i.e. Wegner’s Lemma. ” [Wegner, 1997, Why Interaction Is More Powerful Than Algorithms.]

“Here was mathematical proof that any process that assumed known inputs, like the waterfall method, was doomed to failure when building an object oriented system.” [Schwaber/Beedle, 2002, Agile Software Development with Scrum.]