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In a stroke of irony, this article became more real than initially intended. It was supposed to be published on December 20, 2004 - instead we are now in January 2005. The reason - lack of buffer...
With buffer I mean adding idle time in-between tasks and milestones, so in a case of an unforeseen event, your project plan stays on track. The reason for this is simply that there are many things that can cause a delay:
Project buffers ensure that none of these things will have negative effect on your overall plan.
This one is tricky, because what you can do varies greatly from person to person - and from company to company. In the ideal situation you simple add the project buffers into your project plan.

Ideal project plan with buffer zones
But, some people might take advantage of this situation, and not complete the task on schedule because they think "ohh... there is some extra time". In this case it would be better not to show where the buffer zones are, but add them "secretly" to the project plan.

Project plan with extended tasks for buffer

Project plan with extra buffer before the milestone
Notice: It can be difficult to identify those who are missing deadlines because they are lazy, and those who are missing them because other events interfered.
Well, it depends greatly on the person, the type of work, the uncertainty in the task and the length of the task. In general I would add 50% buffer time to tasks that take only a single day, 20% for tasks that take a week, 10% for tasks that last a month.
You should then increase or decrease these percentages based on the factors above.
Published: Jan. 14, 2005 in Management

Thomas Baekdal is a Writer, Interaction Designer, Change Advocate and Project Manager.