Why Large IT Projects Fail

Large IT projects may not deliver what they promise. Often they are late. Often they are over budget. Often what is eventually delivered is not either what was originally specified nor what is actually needed. There are many reasons why this happens. We discuss some of those reasons here and argue that many of them are a consequence of failure to specify requirements anything like adequately. We introduce the notion of requirements drift as a root cause of failure and show, by the use of models, that the often quite large discrepancies between expectations and reality can be the consequence of quite modest amounts of requirements drift. This preprint is available at http://www.ecs.soton.ac.uk/ ̃ph/LargeIT.pdf