Concept
Planning Fallacy
cognitive-bias optimism-bias planning
People systematically underestimate how long a task will take, how much it will cost, and what could go wrong — even when they have plenty of relevant past experience to draw on. Melina Palmer attributes the root cause to optimism bias: people expect themselves to be faster, better, and more efficient than they were previously, for no real evidenced reason, and fail to account for routine external disruptions (interruptions, other commitments, emergencies) that reliably occur.
A study of 465 undergraduates asked to predict how long their thesis would take found students estimated roughly 28 days on average, with a "best case" of 21 days and a stated "worst case, if everything goes wrong" of 35 days. Actual completion time averaged around 56 days — double the original estimate, and well beyond even the students' own stated worst case (see Buehler Griffin Ross Thesis Planning-Fallacy Study). The effect generalizes broadly: fewer than a third of IT projects meet their original deadline, industrial R&D projects run about 3.5x longer than planned, and up to 90% of professional writers deliver manuscripts late.
Advice for entrepreneurs: set ambitious goals, but limit yourself to no more than about three priorities at a time, and break each into the smallest possible concrete tasks — padding in slack for the external disruptions that are always coming, rather than planning around a best-case scenario.
Beyond individuals, expert forecasting struggles the same way. A study of 300,000 real business forecasts across 8 supply-chain companies found 52% were less accurate than simply predicting no change at all — a related but distinct failure mode to the planning fallacy proper (this is about predicting outcomes, not time estimates), cited by Louis Grenier as a reason to never ask customers to predict their own future behavior either (see Moldridge Business-Forecasting Inaccuracy Study; see also Jobs to Be Done (Clayton Christensen)).
Discussed in
- Episode 45 — 45- 3 Crucial Lessons for Entrepreneurs
- Episode 46 — 46- Five marketing lessons for 2021 (recap)
- Episode 60 — 60- How not to suck at customer interviews (related business-forecasting inaccuracy)