Planning Fallacy
Edited by Paweł Raja, PhD · Published
Definition
Planning Fallacy is the planning fallacy is the tendency to underestimate how long a task will take — and often how much it will cost — even when we know that similar tasks in the past took longer than planned.
- Category:
- Temporal Choice
- Impact:
- High
- Landmark study:
- Kahneman & Tversky (1979)
- Evidence here:
- 5 cited studies
Why it happens: the psychology of planning fallacy
Daniel Kahneman and Amos Tversky (1979) coined the term. The planning fallacy is not simply being late: it is predicting that this time will go to plan while knowing that most similar efforts did not. They traced it to the way forecasts are built. People focus on the specifics of the case at hand — the steps, the plan, the scenario — and neglect distributional information about how cases like it usually turn out.
Kahneman and Lovallo (1993) called these the inside view and the outside view. The inside view constructs a story of how this project will unfold; the outside view ignores the details and asks how long comparable projects took. The inside view feels more informative because it is about this project, but plans are built from the steps you can foresee, and the delays that sink projects are usually the ones nobody planned for.
Buehler, Griffin and Ross (1994) showed the mechanism directly. In think-aloud studies, people predicting their completion times talked mainly about future plans and rarely about their past experience. When they did recall earlier overruns, they attributed them to external, one-off causes that would not recur. When participants were instructed to connect relevant past experiences to their prediction, the optimistic bias disappeared, and observers predicting other people's completion times were not optimistic at all — they overestimated.
Motivation and incentives add to the cognitive error. People want to finish quickly, which makes optimistic scenarios easier to believe, and in organizations an optimistic estimate helps a project get approved. In a study of 258 transportation infrastructure projects, Flyvbjerg, Holm and Buhl (2002) concluded that the cost estimates used to decide whether to build them were highly and systematically misleading, better explained by strategic misrepresentation than by honest error. The planning fallacy is closely related to optimism bias and overconfidence bias, and once a project overruns, the sunk cost fallacy makes it hard to stop.
Planning Fallacy examples
When students writing honors theses were asked for their best estimate of when they would finish, the average prediction was 33.9 days; they actually took 55.5 days on average, and fewer than a third finished by the time they had predicted (Buehler, Griffin & Ross, 1994).
- A software team commits to delivering a feature in a two-week sprint, building the estimate from its task list, and finishes in four — just as the last three features did.
- Allowing 20 minutes for a drive that has never taken less than 30, because you picture the journey with every light green.
- A kitchen renovation budgeted from the contractor's line items runs well over, because nobody budgeted for what opening the walls would reveal.
- A student plans to write an essay over the weekend before the deadline, exactly as they planned — and failed — to do with the last one.
- Leaving a tax return until the final evening because it 'only takes an hour,' a figure that has never once been true.
- Public megaprojects routinely open late and over budget. Buehler, Griffin and Ross (1994) recount that the Sydney Opera House, originally estimated in 1957 to be finished in early 1963 for $7 million, opened in scaled-down form in 1973 at a cost of $102 million.
Planning fallacy and reference class forecasting
The best-supported remedy is to replace the inside view with the outside view. Reference class forecasting, developed by Bent Flyvbjerg from Kahneman and Tversky's work, does this in three steps: identify a class of similar past projects, establish the distribution of outcomes for that class (for example, how far schedules or costs ran over), and place the new project within that distribution, adjusting only for differences you can justify.
The method works because it bypasses both honest optimism and strategic misrepresentation: the forecast comes from what actually happened to comparable projects, not from the plan for this one. The same logic scales down to personal life. If your last five reports each took about a week, the best forecast for the next one is about a week, however tidy the new outline looks.
Planning fallacy vs procrastination
Procrastination is putting off a task you intend to do and is often driven by present bias. The planning fallacy is a prediction error about how long the task will take. They reinforce each other: if you believe a job will take two hours, starting it the night before looks perfectly reasonable.
How to avoid the planning fallacy
- Start from the outside view. Before listing the steps, ask how long the last few similar tasks actually took, and use that as your baseline rather than your plan.
- Connect past experience to the forecast explicitly. Buehler, Griffin and Ross found the optimistic bias disappeared when people were asked to relate relevant past experiences to their prediction.
- Unpack the task. List every sub-step, including reviews, waiting for other people, fixes and handovers; these are the parts plans tend to omit.
- Ask someone else. Observers are not optimistic about other people's completion times in the way we are about our own, so a colleague's estimate is a useful check.
- Track predicted versus actual time for recurring work. Your own records become a personal reference class.
- Set intermediate milestones so that overruns show up early, while there is still time to change scope or ask for help.
How to design for it (nudge strategy)
Ask for estimates anchored on how long similar past projects actually took (reference class forecasting), require a 'what happened last time?' line in every plan, and break long deadlines into near, checkable milestones.
Ethical use: design for choices people would endorse on reflection — a nudge, not sludge. Be transparent and keep opting out easy.
Key studies
Introduced the planning fallacy and attributed it to reliance on case-specific (singular) information over the distribution of outcomes for similar cases, proposing corrective procedures based on that distribution.
Kahneman, D., & Tversky, A. (1979). Intuitive prediction: Biases and corrective procedures. TIMS Studies in Management Science, 12, 313–327.
Students' theses took 55.5 days against a predicted 33.9; people focused on future plans rather than past experience, the bias vanished when they linked past experiences to their predictions, and observers overestimated others' completion times.
Buehler, R., Griffin, D., & Ross, M. (1994). Exploring the "planning fallacy": Why people underestimate their task completion times. Journal of Personality and Social Psychology, 67(3), 366–381. DOI →
Distinguished the 'inside view' (forecasting from the details of the case) from the 'outside view' (forecasting from a class of similar cases) and argued that reliance on the inside view produces bold, optimistic forecasts.
Kahneman, D., & Lovallo, D. (1993). Timid choices and bold forecasts: A cognitive perspective on risk taking. Management Science, 39(1), 17–31. DOI →
Across 258 transportation infrastructure projects worth about US$90 billion, the cost estimates used to approve projects were highly and systematically misleading.
Flyvbjerg, B., Holm, M. S., & Buhl, S. (2002). Underestimating costs in public works projects: Error or lie? Journal of the American Planning Association, 68(3), 279–295. DOI →
Set out reference class forecasting — basing forecasts on the actual performance of comparable projects — and reported its first practical use, for transportation cost forecasts.
Flyvbjerg, B. (2006). From Nobel Prize to project management: Getting risks right. Project Management Journal, 37(3), 5–15. DOI →
Related biases
Planning Fallacy: frequently asked questions
What is the planning fallacy?
The planning fallacy is the tendency to underestimate how long a task will take, and often what it will cost, even when you know that similar tasks have run over before.
Who coined the term planning fallacy?
Daniel Kahneman and Amos Tversky, in a 1979 paper on intuitive prediction. Buehler, Griffin and Ross (1994) later supplied the classic experimental evidence.
What is an example of the planning fallacy?
In one study, students predicted they would finish their honors theses in about 34 days; they took about 56 on average. Everyday examples include underestimating travel time, renovation budgets and how long it takes to write a report.
Why does the planning fallacy happen?
We build forecasts from a plan for this specific task (the inside view) and neglect how similar tasks actually went (the outside view). We also explain away past overruns as one-offs, and we want to finish quickly, which makes optimistic plans easier to believe.
How do you overcome the planning fallacy?
Use reference class forecasting: look up how long or how much comparable projects actually took and start from that figure. Add explicit buffers, list every sub-step and get an outsider's estimate.
What is Hofstadter's law?
A tongue-in-cheek summary of the planning fallacy from Douglas Hofstadter's 1979 book Gödel, Escher, Bach: 'It always takes longer than you expect, even when you take into account Hofstadter's Law.'
Cite this page
Raja, P. (Ed.). (2026, 8 Oct 2026). Planning Fallacy. Behavioral Economics Lab. https://www.behavioraleconomicslab.com/biases/planning-fallacy