Psychologist Roger Buehler's team asked students how long their senior thesis, a long final paper, would take.
They guessed 34 days on average. It took about 56 days.
One explanation: people picture the plan going smoothly. They blame past delays on outside events, so they don't count them as a guide for next time.
The students also guessed a worst case, if everything went badly. Even that fell about a week short.