Patterns of large software systems: failure and success

Software management consultants have something in common with physicians: both are much more likely to be called in when there are serious problems rather than when everything is fine. Examining large software systems-those in excess of 5000 function points (which is roughly 500000 source code statements in a procedural programming language such as Cobol or Fortran)-that are in trouble is very common for management consultants. Unfortunately, the systems are usually already late, over budget, and showing other signs of acute distress before the study begins. The consultant engagements, therefore, serve to correct the problems and salvage the system-if, indeed, salvaging is possible. The failure or cancellation rate of large software systems is over 20 percent. Of those that are completed, about two thirds experience schedule delays and cost overruns that may approach 100 percent. Roughly the same number are plagued by low reliability and quality problems in the first year of deployment. Yet some large systems finish early, meet their budgets, and have few, if any, quality problems. How do these projects succeed, when so many fail?