Estimate duration from similar tasks' actual history, not an ideal plan
Students predicted 33.9 days to finish a thesis but took 55.5; only 29.7% met their own forecast. In another comparison, asking people to recall relevant past experience raised on-time completion from 29.3% to 60.0%.
What it takes
Free. Spend a few minutes checking how long a comparable task really took.
What you may gain
Thesis estimates averaged 33.9 days versus 55.5 actually needed, with only 29.7% finishing within their forecast. In a separate experiment, explicitly connecting past experience to the current task raised completion within the estimated time from 29.3% in controls to 60.0%. Optimistic underestimation disappeared in that condition.
Context & considerations
Buehler used small Canadian student samples: 37 people in one study and about 40 per group in another. Estimating from comparable projects is called reference-class forecasting; Flyvbjerg discusses its engineering application. Halkjelsvik's review found underestimation more common than overestimation, but its size varied considerably by task and method.
Research & references
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 ; 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 ; 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