Why does a job you swore you would finish early always get tight?

When we estimate time, we usually picture how we'll finish the job: find material, write, revise. In that mental movie, interruptions barely appear. Calls, wrong files, and low energy get pushed off the timeline. So the estimate fits the smoothest path, while real time includes every detour. Kahneman and Tversky warned against relying only on that inside view and named the pattern the planning fallacy. An outside record—how long similar work took before—changes the guess, but that record is easy to ignore.

A common myth is that experience alone makes estimates accurate. In a 1994 study, Buehler and colleagues asked thesis students for submission dates. On average, actual completion was later than even their worst-case guesses, and only about 30% met their own predicted deadline. Many had been late before, yet they drew this project as if it would go differently. The past gets filed as "that time had excuses," while today feels like "conditions are good." The pattern repeats, and the forecast shrinks again.

In daily life it helps to note how long similar work took before, not only the steps of this task. Putting a usual timeline and a buffer beside the best-case schedule leaves room for interruptions. Breaking a big job into chunks and attaching past times to each chunk points the same way. Studies differ by task and culture, so the size of the miss varies. Still, the stronger the feeling that "this round will be especially smooth," the more worth doubting that feeling may be.