FallacyEgocentric Bias · Entry 38
Planning Fallacy – when plans make completion times look too optimistic
Planning Fallacy is the tendency to predict that your own task or project will finish sooner than it actually does, even when similar work has taken longer before.
Where can it show up?
- Project estimates: a team builds a detailed plan for smooth execution but gives too little weight to delays in comparable past projects.
- Personal tasks: you picture how writing, studying, moving, or renovating will go if the main steps work as planned, while interruptions and rework receive less attention.
- Delivery reviews: a late project is explained as an unusual case, so the next estimate starts from another fresh plan instead of learning from the history.
A better check
- Start with comparable past work: how long did similar tasks actually take?
- Give a range and name the main sources of uncertainty instead of promising one precise date.
- Keep the original estimate and assumptions so you can compare them with the eventual result.
- Do not use a universal buffer such as ‘multiply by 1.5’. The useful adjustment depends on the task, the data and the reference class.
Evidence review
well-supported for time estimates; size and causes vary by contextWhat the evidence supports
The planning fallacy is a well-documented tendency for people to predict their own task completion times too optimistically. The effect has been observed across different kinds of tasks, but it is not a rule that every plan will run late. Project overruns can also come from changing scope, dependencies, incentives, poor data, deliberate underestimation, or genuinely unusual events, so a late project should not automatically be diagnosed as a planning fallacy.
How researchers describe the pattern
Classic studies found that people often build a detailed scenario for how the current task will unfold while giving too little weight to how long similar tasks actually took. Later reviews describe cognitive, motivational, and social influences rather than one single cause. Recent cross-cultural experiments also suggest that reminders about past experience and the social consequences of delay can change time predictions differently across groups, which is another reason not to treat one mechanism or correction as universal.
Practical interpretation
For an important estimate, start with outcomes from comparable completed work before refining the current plan. Give a range rather than a single precise date, name the main uncertainties, and keep the original estimate so forecast error can be measured later. Reference-class methods can be useful when comparable data exist, but their quality depends on choosing a genuinely relevant class. There is no evidence-based universal rule that every estimate should simply be multiplied by the same fixed buffer.
Reviewed sources
- Exploring the Planning Fallacy: Why People Underestimate Their Task Completion Times experimental studies · 1994 · DOI 10.1037/0022-3514.67.3.366
- The Planning Fallacy: Cognitive, Motivational, and Social Origins research review · 2010 · DOI 10.1016/S0065-2601(10)43001-4
- I Can Do It in No Time! Time Predictions in the Cultural Context cross-cultural experiments · 2026 · DOI 10.1177/00220221261418305
- Reducing risks in megaprojects: The potential of reference class forecasting literature review of reference class forecasting · 2023 · DOI 10.1016/j.plas.2023.100103
Editorial review: 2026-08-18. Evidence status describes this entry, not every study ever published on the topic.
