Ninety–ninety rule

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.

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.

The first 90 percent of the code accounts for the first 90 percent of the development time. The remaining 10 percent of the code accounts for the other 90 percent of the development time.

Tom Cargill, Bell Labs