The "90/90 Rule"

The "90/90 Rule" of project schedules goes like this: the first 90 percent of a project takes 90 percent of the time and effort; the remaining 10 percent of the project takes the other 90 percent of the time and effort.

An aphorism attributed to Tom Cargill of Bell Labs, and popularised by Jon Bentley's September 1985 "Bumper-Sticker Computer Science" column in "Communications of the ACM". It was there called the "Rule of Credibility", a name which seems not to have stuck.