The planning fallacy
People predicting how long a task will take systematically underestimate it, even when they know similar tasks overran, because they reason from the case in front of them instead of from the distribution of similar cases. The corrective is the outside view, which forecasts from the reference class.
Evidence grade: research Peer-reviewed or large-sample empirical work supports it.
Use it when
Explaining why asking people to try harder does not fix estimation; the basis for historical forecasting.
Where it stops working
A reference class has to exist and has to resemble the work. Novel work has none and needs de-risking instead.
Where the standard uses it
- DE-2Estimation, planning, and honest commitmentsManaging uncertainty rather than chasing accuracy.
Sources
- Daniel Kahneman and Amos Tversky. Intuitive Prediction: Biases and Corrective Procedures. TIMS Studies in Management Science, 12, 313–327, 1979. In the bibliography
Used alongside it
- Batch size and predictability practice
- Brooks's law practice
- Cycle time and historical forecasting practice
- The cone of uncertainty practice
Cite this model
Chicago, BibTeX and licence →Engineering Management Academy. (2026). The planning fallacy. In Engineering Management Academy Body of Knowledge (Version 3.1, TEC.planning-fallacy). https://www.engineeringmanagement.academy/body-of-knowledge/techniques/planning-fallacy