The Planning Fallacy

Intermediate

Forecasts for big projects are almost always, and predictably, too optimistic

01 The Concept

The planning fallacy is the systematic tendency to underestimate the time, cost, and risk of future projects, even when the person forecasting has direct, personal knowledge that similar past projects have consistently run over. This happens because planners naturally focus on the specific plan in front of them — the "inside view" — rather than the historical track record of similar projects generally, the "outside view," which is usually the far more accurate predictor.

The fallacy is remarkably resistant to being warned about it directly: knowing about the planning fallacy in the abstract rarely stops a planner from believing their own specific project is the exception, which is exactly why the corrective — deliberately consulting the outside view's track record before finalizing a forecast — has to be built into a process rather than left to individual willpower.

02 Case Study

CASEThe Scottish Parliament building cost overrun, and Kahneman's own curriculum-writing team, both described in *Thinking, Fast and Slow*

The new Scottish Parliament building in Edinburgh was originally estimated to cost up to £40 million; through repeated revisions, the final cost ballooned to roughly £431 million — more than ten times the original estimate. Kahneman offers an even more personal example: while leading a team developing a judgment-and-decision-making curriculum in Israel, he privately asked an experienced curriculum expert on the team how long similar past projects had taken. The expert admitted such teams typically took seven to ten years, with roughly 40% never finishing at all — a stark outside-view warning the team heard, absorbed, and then completely ignored, pressing forward with their own estimate of two years. The actual project took about eight years to complete.

→ Why it matters

Kahneman's own case is the sharper lesson precisely because he was the world's leading expert on this exact bias and still couldn't talk his team (or himself) out of trusting the inside view over direct, specific, credible outside-view evidence sitting right in front of them.

03 Key Points

01The planning fallacy: systematically underestimating time, cost, and risk for future projects

02It persists even when planners know similar past projects have consistently run over

03The 'inside view' (focus on this specific plan) reliably beats out the more accurate 'outside view' (track record of similar projects)

04Knowing about the fallacy abstractly rarely stops someone from believing their own project is the exception

Next: Hindsight Bias: The "I-Knew-It-All-Along" Trap →