While doing projects and literally all tasks from daily to advanced tasks that require estimating time people are inclined to plan with optimistic biases. According to Buehler et al (1994) prediction without render and consider the past and history while estimating time is a common tendency called the planning fallacy.
We tend to underestimate own but not others completion time due to people are inclined to ignore relevance of past experience in present planning of future tasks. Taking into account past experiences and behaviors of failed delivery of tasks in due time in predicting time depend heavily on the perception of how the certain person saw the delay happen.
People make attributions that decrease the relevance of past experience to present planning by thinking the past failed time estimations were for instance of external reasons. They don’t make an excuse. Though, people are likely to not take into account former negative outcomes in prediction purposes. People are so to say explaining away negative outcomes from previous experience.
Do people have to predict other's work they are resorting to more realistic estimations. Then we are including our past experiences in the prediction called distributional information. Buehler et al (1994) distinguish the two roles as actor-observer as two differences in prediction. Observers see actors as procrastinators or sluggish while actors see themselves as subjects to external circumstances while estimating time.
Following the known prediction behaviour above, Buehler et al (1994) carried out five studies to make further evidence for the planning fallacy. The major results were: When people inform present predictions by using and relating past experience they complete more realistic estimates. Reminding of past experiences did not by itself reduce optimistic predictions. Aligned with that fewer than one half in all cases completed their task on predicted time. However, the failed prediction of time varied from one day to weeks depending on the study.
For more information on the studies their article can be read in the following link: