Why Deadlines Slip

Ask people how long a task will take, and they usually underestimate, even when they know that similar tasks took longer in the past. Psychologists call this the planning fallacy. When we plan, we picture the specific steps of the task ahead and imagine them going smoothly; we rarely picture the delays, illnesses, revisions, and interruptions that actually fill a project's timeline. Our past experience does not correct us, because we tend to treat each earlier overrun as a special case caused by unusual bad luck. Large public works are especially prone to this bias, and not only because of honest optimism. A project that appears cheap and quick is easier to approve, so planners may face pressure to present the rosiest possible estimate. One remedy, known as "reference class forecasting," asks planners to set aside the details of their own project and look first at how long similar projects actually took. If most comparable bridges ran thirty percent over schedule, that figure becomes the starting point, adjusted only for genuine differences. The method feels pessimistic, but its users point out that a realistic estimate is not pessimism; it is simply the average experience of everyone who tried the same thing before.