The Ninety-Ninety Rule

This rule quantifies the extent to which projects get stuck in the final phase. Often, teams make good progress at the start, building core functionality, which leads to optimism. Then integration, corner cases, performance tuning, and bug fixing consume an unexpectedly large amount of time, often equal to or greater than what has already been spent.

In practical terms, this rule advises that the last bit of a project is not a small bit. You should plan for a significant effort in finishing, polishing, and delivering.

It is also related to Hofstadter’s Law, as it is another way our estimates fall short. We do not realize how hard that last leg is.

• The final parts of a project (polishing, edge cases, integration, bug fixes) often take far more effort than anticipated, often as much as the initial development.
• What looks like the 'finishing touches' involves many tricky problems and unknowns.
• Reaching a 'mostly done' state can create optimism, but the rule warns that 'almost done' often means only halfway in terms of time.