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As a rule, software systems do not work well until they have been used, and have failed repeatedly, in real applications.
The picture of the software designer deriving his design in a rational, error-free way from a statement of requirements is quite unrealistic. No system has ever been built that way, and probably none ever will. Even the small program developments shown in textbooks and papers are unreal. They have been revised and polished until the author has shown us what he wishes he had done, not what actually did happen.
David Parnas and Paul Clements Quoted in Code Complete, Chapter 7: High-Level Design in Construction.
Code Complete: A Practical Handbook of Software Construction by Steve C McConnell
ISBN: 1556154844 This book is available from Amazon
