Ninety-Ninety Rule

In computer programming and software engineering, the ninety-ninety rule states:

"The first 90% of the code accounts for the first 90% 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).