In computer programming and software engineering, the ninety-ninety rule is a humorous aphorism that states:
This adds up to 180%, making a wry allusion to the notoriety of software development projects significantly over-running their schedules (see software development effort estimation). The anecdote expresses both the rough allocation of time to easy and hard portions of a programming undertaking, and the cause of the lateness of many projects in their failure to anticipate their difficult, often unpredictable, complexities. In short, it often takes both more time and more coding than expected to complete a project.
The rule is attributed to Tom Cargill of Bell Labs, and was made popular by Jon Bentley's September 1985 "Programming Pearls" column in Communications of the ACM, in which it was titled the "Rule of Credibility".