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Life Guide U.S. essentials
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Estimate how long a task will take from how long similar tasks actually took, not from your plan

  • Top value
  • Evidence B
  • Saves time
  • Free
  • Takes minutes
  • No willpower

In plain words

Students estimated they would finish their thesis in 33.9 days. It actually took 55.5 days, and only 29.7% finished in the time they had estimated. Another group was asked to recall how long the last similar task took before estimating, and the share who finished on time rose from 29.3% to 60.0%.

Cost

Free. Before estimating, spend a few minutes looking up how long the last similar task actually took.

What you get

Students expected to finish a thesis in 33.9 days and actually took 55.5 days. Only 29.7% finished on time by their own estimate. The researchers asked another group to link past experience to the task at hand before estimating. In that group the share finishing on time rose from 29.3% in the control group to 60.0%. The optimism that makes people underestimate how long a task will take no longer appeared in this group.

Notes

The study by Buehler recruited only Canadian university students. It is a single study with few people: 37 people, and about 40 per group. Estimating from the historical data of similar projects is called reference class forecasting; for how it is done in practice in engineering, see Flyvbjerg. After pooling several studies, Halkjelsvik pointed out that underestimating is more common than overestimating, but by how much varies a great deal with the task and the research method.

Evidence grade
B : Supported by research but without a precise number, or based on small or single studies.
Sources
  1. 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. https://doi.org/10.1037/0022-3514.67.3.366
  2. Flyvbjerg, B. (2006). From Nobel Prize to project management: Getting risks right. Project Management Journal, 37(3), 5–15. https://doi.org/10.1177/875697280603700302
  3. Halkjelsvik, T., & Jørgensen, M. (2012). From origami to software development: A review of studies on judgment-based predictions of performance time. Psychological Bulletin, 138(2), 238–271. https://doi.org/10.1037/a0025996

Translated from the Chinese original. Original book, section 4, tip 4. Content may be out of date and rules differ by place. When you actually deal with an agency, a doctor or a court, follow what local officials, doctors and lawyers tell you. Disclaimer