13% of subjects finished their project by the time they had assigned a 50% probability level; 19% finished by the time assigned a 75% probability level; and only 45% (less than half!) finished by the time of their 99% probability level. As Buehler et. al. (2002) wrote, "The results for the 99% probability level are especially striking: Even when asked to make a highly conservative forecast, a prediction that they felt virtually certain that they would fulfill, students' confidence in their time estimates far exceeded their accomplishments."

I do agree that it is good to have a pessimistic view on projects over an optimistic view because then you will finishi sooner but if you have in mind that at best I will finish two weeks before the deadline and at worst the deadline day then you might be happier working on the project because you think to yourself that the project could be almost done. What is most important is to be honest about when you can be done because if you say that you might be done at a time that you for sure won't be done then you work on the project thinking inside your head that I will be finished later than I said. Optimistic vs. Pessimistic view is a balance between being happier and finishing sooner.