IT Project Management, 2nd Edition

7.8. Management Reserve

You and your project team will no doubt be tempted during the creation of each task to overstate the estimated amount of time for it to be completed. Don't yield to this temptation. Always reflect the accurate amount of time it should take to complete a task. You can use PERT's optimistic, pessimistic, and most likely approach if you'd like, but don't inflate the time required for each task to allow for a mistakes, rework, and late activities within the project.

The reason is explained in Parkinson's Law. Parkinson's Law states that work will expand to the fill amount of time allotted to it. In other words, if your project team says an activity will take them 24 hours to complete, but they know the work will probably ...