The planning fallacy is a cognitive bias where individuals underestimate the time required to complete future tasks despite knowing that similar past tasks took longer. This tendency leads to overly optimistic projections about task completion, even when one understands that past estimates were inaccurate. The bias particularly affects one's own tasks, contrasting sharply with outside observers who tend to overestimate the time needed for the same tasks.
In a study, psychology students predicted they would complete their senior thesis in an average of 33.9 days, but the actual average completion time was 55.5 days, showing a clear underestimation of time needed.
To counteract the planning fallacy, break tasks into smaller components and apply both optimistic and pessimistic time estimates to encourage a more balanced approach.