Ninety-ninety rule

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In computer programming and software engineering, the ninety-ninety rule is a humorous aphorism that states:

That the total development time sums to 180% is a wry allusion to the notorious tendency of software development projects to significantly overrun their original schedules. 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). In other words, it takes both more time and more coding than expected to make a project work.

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.[1]


Because the percentages do not sum to 100, Cargill's aphorism is sometimes mistaken for a typo. The "corrected" version of the rule is sometimes quoted as:

Another variation appears as:

A less common variant is changing all percentages to 90%:

A separate variation refers to a different metric and is applicable to projects other than software-related:

See also


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