A quote from Dynamics of Software Development Jim McCarthy & Michele McCarthy ISBN: 0-7356-2319-8 http://www.microsoft.com/learning/en/us/book.aspx?ID=9691 ​

Doctors mostly manage to retain a modicum of respect and social approbation without pandering to the fears and insecurities of their patients, all of whom are destined to die. Theirs is a good model for setting expectations. Software development, like medicine, is not an exact science. It’s unfortunate that software development isn’t yet generally recognized as an art form that happens to have technical aspects. ​For now, we really have to learn to be like the doctors. They are able to say, quite comfortably and confidently and with conviction, “These things are never certain.” Doctors seldom if ever state with certainty what the outcome of any procedure might be. Yet software managers, operating in a far less disciplined and less data-driven environment, one with much less history and of a complexity perhaps equal to the complexity of the anatomical systems that are the doctor’s purview, blithely promise features, dates, and outcomes not especially susceptible to prediction. Worse, doctors are usually dealing with a subsystem that is malfunctioning While the overall system remains largely functional. In software development, the goal is often a wholly new system that has never functioned before at all.