Planning Fallacy

What is the planning fallacy?

The planning fallacy describes overly optimistic predictions about how long one’s task will take, particularly when those predictions give too little weight to relevant past experience. The term is also used more broadly for underestimated project demands.

In Buehler, Griffin, and Ross’s studies, participants often focused on how a task would unfold rather than how similar tasks had gone. In one study, asking them to connect relevant past experience to their forecast eliminated the observed optimistic bias.

Planning fallacy example

For example, expecting to complete an assignment in one evening despite repeatedly needing several evenings would illustrate the pattern. A single late completion is not sufficient evidence: unexpected events can delay even a reasonable forecast.

How to address the planning fallacy

Compare the plan with records of similar completed tasks. How long did they actually take, and what makes this one different? Those records can inform a forecast; a general instruction to “be realistic” cannot guarantee accuracy.