Relevant to the HN crowd and a helpful explainer of at least one psychological reason why projects are often underestimated:
"In one series of studies, people predicted how quickly they (or others) would complete various work projects and whether they would meet their deadlines. They were overoptimistic about themselves (but not others) because they focused on their industrious motives and intentions rather than their past behavior or the behavior of others in similar situations (6)."
I know that, historically, I personally _always_ forget to take interruptions and the time spent context switching for such interruptions, which can dwarf even well estimated efforts.
I heard somewhere that when a task becomes too big for people to truly estimate the actual time-taken, they unconsciously substitute their confidence that it can be done.
I take into account my internal desires and intentions
You have positive illusions
He is biased
* a character from "Yes, Minister":
That’s one of those irregular verbs, isn’t it? I give confidential security briefings. You leak. He has been charged under section 2a of the Official Secrets Act.
"In one series of studies, people predicted how quickly they (or others) would complete various work projects and whether they would meet their deadlines. They were overoptimistic about themselves (but not others) because they focused on their industrious motives and intentions rather than their past behavior or the behavior of others in similar situations (6)."