It is sometimes said that if you want a task to be done, you have to ask someone very busy to do it! Apparently, busy people have so little time that they have to be efficient and do things without waiting. These people probably know how to manage their time. But this ability to manage time efficiently, without procrastination, requires at least a good estimate of the time required to complete a task. So, to estimate the time that a task will take, it is still necessary to know this task. This knowledge might include the need to remember how much time performing a similar task had taken, or normally takes.