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"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".
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.