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The “ninety-ninety” rule states that the first 90 percent of code construction uses up 90 percent of development time, and that the remaining 10 percent of code construction accounts for another 90 percent. With a total adding up to 180 percent, this IT saying is clearly satirical.
It has sometimes been called the “rule of credibility” and often attributed to Tom Cargill of Bell Labs, or John Bentley’s articles in “Communications of the ACM.”
The idea behind the “ninety-ninety” rule is that in the first 90 percent of a project, code construction takes place in a steady and linear way. The last 10 percent of construction is often where more challenges might present themselves, for instance, in debugging, fixing tougher sets of features, or putting the finishing touches on a project. There is the idea that the home stretch is where projects become really difficult, and start to suffer from time bloat. Another way to put this is that projects tend to be “resistant to ultimate signoff” when they are nearing completion.