Ninety-Ninety Rule
90-90 Rule
The adage, often in computer programming, that the first 90 percent of the code in a project accounts for the first 90 percent of the development time, and the remaining 10 percent of the code accounts for the other 90 percent of the development time. In other words, the tendency to underestimate the amount of time to complete a project when it is "nearly done".
Origin
Attributed to Tom Cargill, a computer programmer and consultant at Bell Labs, who coined the rule in the mid-1980s as a humorous observation about software development timeline challenges, formulated from his practical experiences with complex C programming projects. The rule was popularized by Jon Bentley in his September 1985 "Bumper-Sticker" column in Communications of the ACM, exposing it to thousands of computing professionals worldwide. The rule highlights how the last 10% of development—bug fixes, optimization, final adjustments—proves unexpectedly time-consuming, causing projects to take roughly 180% of initially estimated time.