A Time Buffer is not the same as Total Float
BY STIJN VAN DE VONDER | IN PRIMANED BLOG | 8 MARCH 2018
Who owns the float has been a predominant discussion within project controls for decades. And it still is. However, to correctly address this question, the difference between Total Float and Time Contingency (or a time buffer) needs to be fully understood. We hope this blog post will help in clarifying the differences.
Total Float is nothing else than the difference between the Early Finish Date and the Late Finish Date (or Early and Late Start) of an activity as calculated by the Critical Path Method.
It is a scheduling thing. It denotes how much an activity can slip without impacting some constraints, and often more relevant, any contractual milestone restricted by such constraint. Total Float shows that there is some sort of abundance in the planning. The time that is realistically estimated to complete a group of activities is smaller than the time allowed in the contract to complete them. This total float can be consumed without severe consequences for the project. But what if two contractual parties have suffered delays and both want to use the total float to mitigate them? Then the ownership question arises. We will come back to this later in this post.
A time buffer is something completely different. It is a contingency reserve added to the schedule to cope with existing risks and uncertainties. Mostly contingency reserves are added in front of major milestones to protect these milestones. This is the reasoning behind a project owner requesting its contractors to show that deadlines are met with a probability of at least 80%. The size of the required time buffer to obtain this kind of robustness should always be calculated based on underlying risks and uncertainties. Running a schedule risk analysis is the most correct way of calculating it. The required time contingency is then for example calculated as the difference between the P80 finish date of a contractual milestone and its finish date in the deterministic schedule. The resulting time buffer can then be added to the baseline schedule as an activity after the last activity to be completed before the milestone. After that, a risk-adjusted schedule can be calculated by re-applying the critical path method. Total float will be updated and will probably be reduced for the key milestones.
Tags: Primavera , Planning