# Planning fallacy The planning fallacy is our tendency to underestimate how long our own tasks and projects will take, even when we know that similar tasks ran late in the past. [[Daniel Kahneman]] and [[Amos Tversky]] named it in 1979. Lovallo and Kahneman later widened the definition to include underestimated costs and risks and overestimated benefits. The best-known evidence comes from Buehler, Griffin and Ross (1994). Psychology students predicted when they would finish their senior thesis: on average 33.9 days, and 48.6 days even in a "worst case" scenario. They actually took 55.5 days, and only about 30% finished within their own estimate. The same studies showed three things worth remembering: - The bias applies to our own tasks. Observers predicting someone else's completion time were too pessimistic, not too optimistic - We plan from a best-case scenario of the future and ignore our track record. When we explain past delays, we treat them as one-off, external accidents - Asking people to explicitly connect their past experience to the new prediction removed the bias in one study Why it matters: every deadline, sprint and weekly plan we set is exposed to it. The fix is not to try harder to be realistic, but to change the method: estimate from what similar work actually took ([[Reference class forecasting]]), track estimated versus actual time, and add buffers. See also [[Hofstadter's Law]] for the tongue-in-cheek version. ## 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 - Kahneman, D., & Tversky, A. (1979). Intuitive prediction: Biases and corrective procedures. TIMS Studies in Management Science, 12, 313-327 (no stable URL) - Lovallo, D., & Kahneman, D. (2003). Delusions of success: How optimism undermines executives' decisions. Harvard Business Review. https://hbr.org/2003/07/delusions-of-success-how-optimism-undermines-executives-decisions - https://en.wikipedia.org/wiki/Planning_fallacy - https://concepts.dsebastien.net/concept/planning-fallacy/ ## Related - [[Reference class forecasting]] - [[Hofstadter's Law]] - [[Parkinson's Law]] - [[Escalation of commitment]] - [[Cognitive biases]] - [[Thinking Fast and Slow (book)]] - [[Project Management (MoC)]]