In computer programming and software engineering

Software engineering

Software Engineering is the application of a systematic, disciplined, quantifiable approach to the development, operation, and maintenance of software, and the study of these approaches; that is, the application of engineering to software...

, the ninety-ninety rule is a humorous aphorism

Aphorism

An aphorism is an original thought, spoken or written in a laconic and memorable form.The term was first used in the Aphorisms of Hippocrates...

 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

Bell Labs

Bell Laboratories is the research and development subsidiary of the French-owned Alcatel-Lucent and previously of the American Telephone & Telegraph Company , half-owned through its Western Electric manufacturing subsidiary.Bell Laboratories operates its...

 and was made popular by Jon Bentley's

Jon Bentley

Jon Louis Bentley is a researcher in the field of computer science. He is credited with the invention of the k-d tree....

 September 1985 "Programming Pearls" column in Communications of the ACM

Communications of the ACM

Communications of the ACM is the flagship monthly journal of the Association for Computing Machinery . First published in 1957, CACM is sent to all ACM members, currently numbering about 80,000. The articles are intended for readers with backgrounds in all areas of computer science and information...

, in which it was titled the "Rule of Credibility".