Body of Knowledge · Glossary
Planning fallacy
The systematic tendency to underestimate how long a task will take, even when similar tasks are known to have overrun, because the forecast reasons from the case at hand instead of from the distribution of similar cases. The outside view corrects it.
Where the standard develops it
- DE-2Estimation, planning, and honest commitmentsManaging uncertainty rather than chasing accuracy.
Related terms
- Brooks's law
- Adding people to a late software project makes it later, because the existing people must train the new ones and the communication paths multiply.
- Commitment versus estimate
- An estimate is the best forecast.
- Cone of uncertainty
- Estimates are least accurate at the start of work, when least is known, and narrow as reality is discovered.
- Cycle time
- How long work takes from start to done, as measured.
Cite this definition
Chicago, BibTeX and licence →Engineering Management Academy. (2026). Planning fallacy. In Engineering Management Academy Body of Knowledge (Version 3.1, GLO.planning-fallacy). https://www.engineeringmanagement.academy/body-of-knowledge/glossary/planning-fallacy