Pedro Arantes

The 90-90 Rule

When you finished 90% of any project, the remaining 10% will take the same amount of time of the fist 90%.
Zettelkasten,February 07, 2021 (updated at March 08, 2021)


  • "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.

  • "Be prepared: When you are 90% done with any large project (a house, a film, an event, an app), the rest of the myriad details will take a second 90% to complete." Kevin Kelly.

  • When you finished 90% of any project, the remaining 10% will take the same amount of time of the fist 90%.

  • It's always add more time to any software time estimation.

  • This happens because:

    • We don't account unexpected events. We may think that the time is linear - the first 90% takes 90% of the time, then the final 10% will take 10% of the time - and because Parkinson's Law, we project the whole time to the expected tasks. This leaves us with a little time to unexpected.

    • The first 90% may be the tasks that activate the flow state and the last 10%, the boring tasks.


  • What was the last time you saw this rule happening?

  • How many kinds of projects this rule may be applied?

  • Does setting an aggressive deadline minimizes the 90-90 effect? If yes, why?


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