Planning Fallacy
Type: Cognitive Bias
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Definition
Underestimating time, costs, and risks of future projects while overestimating benefits. We plan for best case, reality delivers average case.
Kahneman & Tversky (1979): Students estimated thesis completion time. Average estimate: 34 days. Average actual: 56 days. Only 30% finished by their estimate.
Why It Matters
Projects: Perpetually late, over budget. Software: “Two weeks” becomes two months. Travel: “I’ll be there in 10 minutes” — You won’t. Government: Big projects (tunnels, IT systems) routinely 2-3x over budget.
Causes
- Optimism — “It’ll go smoothly for ME”
- Anchoring — Initial estimate becomes anchor
- Inside view — Focus on this task, ignore historical data
- Social pressure — Promise aggressive timelines
Fighting It
- Reference class forecasting — How long did similar projects take?
- Add buffer — Multiply estimate by 1.5-2x
- Pre-mortem — “Imagine it’s failed. Why?”
- Break it down — Small tasks are easier to estimate
- Track and adjust — Compare estimates to reality
Related Biases
- Optimism Bias — “It won’t happen to me”
- Anchoring Bias — First estimate sticks
- Overconfidence Effect — We know more than we do
Audio
Podcast episode: Planning Fallacy
Part of the Cognitive Bias Reference