Planning fallacy

Estimation biases biases

The planning fallacy is the tendency to underestimate how long a task will take or how much it will cost, even when you have experience of similar tasks taking longer. We focus on the plan and best case instead of the typical outcome, so deadlines and budgets are often too optimistic.

Examples

  • You assume you can finish the report in one morning because that's the plan, even though similar reports have always taken two days.

  • You budget two hours for a journey you have made before, forgetting that traffic or delays usually make it three.

  • A team estimates a project will be done in six months; similar projects in the same company have usually taken a year.

  • You plan to declutter the garage in an afternoon, then find it takes a full weekend once you start.

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