The planning fallacy: why projects usually take longer than the estimate
A famous study of the planning fallacy: even the worst-case estimate was too short
Thesis students gave three finish dates: if all went well, a best guess, and if everything went badly. Most of them missed even the worst one. Why do people who know they run late guess short again?
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1
In 1994, the psychologist Roger Buehler and two colleagues found students who were writing their thesis, the long paper that ends their studies. They asked them one thing: when will you finish?
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2
Each student gave three answers: a date if all went well, a best guess, and a date if everything went badly. On average, 27, 34 and 49 days.
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3
On average, they needed 56 days. Fewer than a third were done by their best guess. Fewer than half were done even by their worst case.
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Students like these said that about two in three of their past projects had finished later than expected. Yet in later tests by the same team, students guessing a date thought about the plan ahead and hardly ever about their past. Asked about old delays, they blamed one-offs, like a flu or a broken laptop.
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5
Each delay looked like bad luck of its own. Added up, the bad luck was the usual case, and the next guess came out short again.
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Quick check
Will she be on time tonight?
Show the answer
Probably not. Each reason was different, but together they show she is late most times, and her promise comes from tonight's plan, not from her past.
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Go further
The study, if you want to read it.