Planning Fallacy
Teams consistently underestimate how long things will take — and they do it even when they know about this pattern.
Why PMs should care
When a team looks at its own project, every feature feels doable, every risk feels manageable, and every dependency feels like 'a quick chat with the other team'. This optimism is almost impossible to correct from inside the project.
The fix isn't psychological; it's structural. Replace the inside view with the outside view. Instead of asking 'how long will this take?', ask 'how long did the last five things that looked like this take at this company?' The outside view is almost always 30–60% longer than the inside estimate — and it's almost always right.
The hard part isn't doing this calculation. It's having the backbone to commit to the outside-view number in a planning meeting where everyone else is still working with the optimistic one. That's where the real skill is.
Example in product work
A PM is scoping a KYC periodic review project. The team's gut estimate: 'six weeks, we've done this before.'
The PM pulls up Jira for the three most recent KYC-adjacent projects:
— Periodic refresh: 14 weeks
— EDD flow rebuild: 16 weeks
— Sanctions screening POC: 12 weeks
The outside-view average is 14 weeks — more than 2× the inside estimate. The room pushes back: 'but this one is simpler'. It's always simpler.
The PM commits to 12 weeks with a checkpoint at 8, instead of 6 weeks with a sprint of 'finishing touches' — and survives the quarter.
What to do when you see it
- Optimism about your own project is almost impossible to correct from the inside.
- Instead of asking 'how long will this take?', ask 'how long did the last five similar things take?'
- The outside view is usually 30–60% longer than the team's gut estimate — and it's usually right.
- The hard part isn't finding the honest number. It's committing to it in a meeting where everyone else is still using the optimistic one.
Sources & further reading
- Intuitive Prediction: Biases and Corrective Procedures — Kahneman & Tversky, 1977The original paper introducing the planning fallacy.
- Thinking, Fast and Slow — Daniel KahnemanKahneman's book-length treatment, with a chapter dedicated to the planning fallacy and reference class forecasting.