MENTAL MODEL · M2612

The 90-90 Rule

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Version 1.0.0 · Updated 2026-07-30

CORE DEFINITION

"The first 90% of the code takes the first 90% of the development time. The remaining 10% of the code takes the other 90% of the development time." -

SCAFFOLDING EFFECT

psychology

Reduce cognitive load

The deceptive nature of progress bars. - This is a humorous recursive law. When you feel the project is "almost done," you are actually only halfway through. Because the final debugging, compatibility, and detail polishing often take more time than core feature development. When scheduling, if a task is 90% complete, be sure to tell your boss: "We are just over halfway."

anchor

Anchor fast decisions

Based on "nonlinear finishing" and "hidden workload." Core feature development is easily overestimated in progress, while debugging/integration/details take up the majority, leading to "90% complete actually means halfway."

MINIMUM ACTION

In progress 0/1

Practice this model in one real situation:

Estimate the "finishing" phase separately in scheduling. 2. When progress reaches 90%, still manage with a halfway mindset. 3. Reserve buffer for integration and testing. 4. Use Definition of Done (DoD) rather than "code written."

Check to track your progress (stored locally)

Learning progress0%

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Source support: Explicit

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zh.wikipedia.orghttps://zh.wikipedia.org/wiki/90-90%E6%B3%95%E5%88%99ZH · Explicit
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