Why Do We Underestimate How Long Our Own Tasks Will Take?

When preparing a report, it is easy to imagine a smooth sequence: gather sources, write a draft, revise, and finish in two days. A similar report may have taken five days last time, yet this one still feels different: ‘I know what I’m doing now.’ This persistent optimism about the completion time of our own tasks is commonly called the planning fallacy.

Research by Buehler, Griffin and Ross found that people estimating their own tasks focused mainly on how the current plan would unfold and made less use of relevant past experience. Earlier delays were also readily explained as one-off external events: an unexpected meeting, a computer failure or a slow reply from somebody else. Each explanation may be true. If similar ‘exceptions’ keep recurring, however, they are part of the task environment.

The problem is not simply optimism in general, and it is not the same as procrastination. Someone may start immediately while still underestimating the time required for searching, task-switching and rework. Conversely, one delay caused by a genuinely rare accident does not establish a stable bias. Early studies often used student assignments and everyday projects. They demonstrate a repeatable tendency in judgement, not an identical effect for every person and task.

A better correction is not merely to order yourself to ‘be more pessimistic’, but to change the information behind the estimate. Find one or two genuinely comparable tasks and examine how long they took from start to delivery, then identify any verifiable differences this time. Estimating the stages separately also tends to produce a longer allocation than naming one total. The aim is not to add an arbitrary buffer. It is to let past reality enter the present judgement.

https://web.mit.edu/curhan/www/docs/Articles/biases/67_J_Personality_and_Social_Psychology_366%2C_1994.pdf
https://pubmed.ncbi.nlm.nih.gov/18604961/