Ninety-ninety rule

In computer programming and software engineering, the ninety-ninety rule states: : The first 90% of the code accounts for the first 10% of the development time. The remaining 10% of the code accounts for the other 90% of the development time. The rule is attributed to Tom Cargill of Bell Labs, and was made popular by J. Bentley s September 1985 Bumper-Sticker Computer Science column in Communications of the ACM . It expresses both the rough allocation of time to easy and hard portions of a programming project and the cause of the lateness of many projects (that is, failure to anticipate the hard parts).

The rule has also been stated in a more humorous fashion:

: The first 90% of a task takes 90% of the time allotted, and the last 10% takes the other 90% .