Safe Float – what is it and how does it impact the risk analysis process?
Let’s just delve right in to this. In the model below we can see that there are three activities with finish to start relationships. The duration for each activity is 30 days. There is a 10 day gap between each activity (this is a feature of GPM based on planned dates) The float value is displayed beneath each activity with 54 for Activity (A).
This is a deterministic schedule showing that activity (C) will end on July 11. However in a risk assessment one would assign an optimistic, pessimistic, and most likely value to each of these three activities. For our purposes we will assign an optimistic value of 20 days and a pessimistic value of 40 days to each of the activities.
In order to determine the safe float envelope we need to apply the pessimistic duration to each activity. If we do this each activity will be 40 days long.
The remaining float in this case is 24 days for each activity. This means, our safe float would be 24 in total for the chain. We know that we cannot exceed the project completion date if we float this chain, in total 24 days or less.
This is true because in the simulation, each iteration has the potential to select the pessimistic date for every activity but even then we still have 24 days of float or “safe float”
If we are simultaneously floating activities in the simulation without regard to safe float, we would have the potential to push past the deterministic date, with our floating operation in conjunction with the pessimistic simulation.
Obviously we would never choose to float an activity if we thought it would push us past our projected project end date. In order to be SAFE we need to limit our floating of activities within the safe float envelope.