Estimating task durations can be extremely difficult to grasp, if we were to ask several project team member how long will it take to write the same report I guarantee we would get several different answers.
Why? Some people will have the thought process of it should take me 2 days at 100 % of my time to write, however there is other work I have to do, so they add another day, plus an extra one to allow for other work ! By the time they give you there estimation a 2 day task turns into a 5 day one! Add the number of tasks up in your plan and think about how much extra time has been added and whether or not you really have a true indication of how long the project will take.
Now if we think about the actual work getting completed many people will look at the schedule and due dates and think they have enough time up their sleeve and delay starting the task until the last minute! Well this is student syndrome working at its best, but if the estimations are incorrect - be prepared to go off to your Sponsors and explain why your project is behind time.
So how do we reduce the impacts of this , we can apply several methods to assist us with estimation such as past experiences , Theory of Constraint or Monte Carlo Analysis , the list is endless. However one of the most effective ways is to ensure your project team are cohesive and explain the value of estimation to them, allow people to think about durations on tasks and the importance of those estimates when producing your project plans.