What It Means
The planning fallacy is the tendency to underestimate how long a task or project will take, even when you've seen similar tasks run late before. Psychologists Daniel Kahneman and Amos Tversky named it and traced it to looking only at the details of the job in front of you instead of at how similar jobs actually turned out. Later work widened it to include underestimating costs and risks as well as time.
Try It: Adjust an Estimate by Your Own Track Record
Add up a few similar past jobs: what you guessed and what they really took. The calculator scales a new estimate by that record. The numbers shown are an example.
Your past jobs ran long, so plan the new one at the adjusted number.
Your past jobs finished on time or early. Keep checking as you add more.
How People Say It
- "It's a quick job, maybe an hour."
- "Every kitchen remodel takes twice as long as they say."
- "I always think I'll finish the report by lunch."
The Breakdown
Worked example (Buhata's own). Three recent weekend jobs, estimated and then timed:
| Job | Estimated | Actual |
|---|---|---|
| Painting the bedroom | 4 hours | 6 hours |
| Doing the taxes | 3 hours | 5 hours |
| Clearing the garage | 3 hours | 3 hours |
| Total | 1010,000 stepsA popular daily step goal that began as the name of a 1960s Japanese pedometer; later studies find health benefits begin well below it. See more hours | 14 hours |
14 ÷ 10 = 1.4, so these jobs took 1.4 times the estimate, a 40% overrun. A new job you'd guess at 3 hours becomes 3 × 1.4 = 4.2 hours, about 4 hours and 12 minutes.
What the result means. A number above 1 says your own guesses have run short, and the adjusted estimate is the one to put on the calendar. The more past jobs you add, the steadier the ratio gets. This is the practical side of what Kahneman called taking the outside view: judging a plan by how similar cases went instead of by how this one feels.
Where It Comes From
Psychologists Daniel Kahneman and Amos Tversky named the planning fallacy. Bent Flyvbjerg's paper on project management, which credits them with the term, describes the fix in Kahneman's words: trading the "inside view," where a planner focuses on the specifics of the task, for the "outside view," which looks at how similar past cases turned out. Flyvbjerg and others later applied it to large projects and their budgetsBudgetA plan for a set period, usually a month, that lists the money you expect to receive and how you'll spend or save it. See more.
Good to Know
Knowing about the planning fallacy doesn't switch it off; the original definition covers people who have seen past jobs run late. What helps is writing down your estimate before you start and timing the job, so you build your own record to plan from. A Focus timer or a note of start and end times is enough for this.
FAQ
What is an example of the planning fallacy?
Guessing that painting a room will take four hours when your last two rooms each took six. The planning fallacy is making that short guess even though you've seen the overrun before.
How do you avoid the planning fallacy?
Base the estimate on how long similar jobs really took, which Kahneman called taking the outside view. Timing a few tasks and multiplying new estimates by your own overrun ratio is one simple way.
Who came up with the planning fallacy?
Psychologists Daniel Kahneman and Amos Tversky, who described the tendency to underestimate completion times.
Do It in Buhata
Tasks and Focus
Time your next few jobs with Focus in Buhata, note the estimate and the real time on each item in Tasks, and use the ratio for your next plan.
Open BuhataRelated Terms and Categories
- Hofstadter's law
Douglas Hofstadter's joke rule from his 1979 book Gödel, Escher, Bach: a task always takes longer than you expect, even after you allow for this rule.
- Time tracking
Recording how much time goes to each task or type of work, either to see where your hours really go or to keep the hours records pay depends on.
Sources
"The Planning Fallacy in the Orthopedic Operating Room" (PubMed Central); Markus K. Brunnermeier and colleagues, "An Economic Model of The Planning Fallacy" (Princeton University); Bent Flyvbjerg, "Quality Control and Due Diligence in Project Management: Getting Decisions Right by Taking the Outside View" (arXiv)
Last updated .