Estimating how long something takes is always a problem, especially when you factor in the external "influences" the person doing the work has to navigate within the organization.
The other factor is the overriding culture of the organization. Is it a learning culture that stimulates open and honest discussion or is it a punishing organization where it is better not to fail, rather than taking risks that might lead to failure (i.e., playing not to lose vs playing to win)?
How many of you reading this have experienced the "Can't you do better with your estimate?" "Can't you raise the bar for the team and 'motivate' them to be more aggressive?" or "Plan for success not failure..." and so on.
So the engineer makes the estimate of 8 days to do something. Of course she is the subject matter expert and has done it many times, so she estimates 8 days considering the unknowns of the problem she is trying to solve and that it has not been clearly defined.
Her manager the Director of Eng says, "I have done it before in 6 days, so make it 6 days." When he presents to his boss, the VP of Eng, he gets hammered for padding the schedule, so he gets backed into a corner and says, "It can be done in 4 days, 'no problem.'"
When the VP of Engineering presents the schedule to the CEO, at the weekly project review, he is challenged by the CEO who says that, "There is no way that this should take more than 1 day, because it must go faster given the strategic importance it has to the company." In the background the VP of Marketing is frustrated because she already promised it to the customer. She told them last week "It was already done."
Later that day the Engineer is informed that her task will take 1 day. 8 days from now she reports that she is not done and may need another 2 days to finish it. It took her some time to define the problem clearly before exploring solutions, as she had predicted. Everyone up the chain of command is shocked and surprised! No one can understand why they have "execution problems" in the Engineering group.
No wonder people don't take schedules seriously. Also it is a good example of why the people that actually have to do the work don't really "buy in" to schedules and the implied contract when they are told what "their" schedule is.
Recently, I rewatched one of my favorite movies, "The Bridge on the River Kwai." I must have seen it ten times. If you get a chance to watch it again, observe what happens when the Japanese camp commander tells the British POWs when the bridge "will be finished" and is not interested in their input, even though many of the British officers are experts in bridge construction and he nor anyone on his staff has done a similar bridge project before. His focus is his deadline, therefore this is how long it must take. Any information to the contrary is rejected.
When the British commander, Sir Alex Guinness, objects he is placed in a hot box for days and his officers are made to stand for the day in the hot sun without water. Basically, the British enlisted men (minus their officers) revolt by slowing down the project and screwing up the construction.