Negative Float appears in a schedule when the early dates of an activity are later than the late dates. It typically indicates a missed date somewhere along the Critical Path.
In nearly every case where there is Negative Float in a schedule, it will be due to a Constraint somewhere on the Critical Path. This is because constraints place hard, early or late dates on activities.
In the following Activity node, shown below Figure 1, we can see that the late dates are five days earlier than the early dates.
During a forward and backward critical pass calculation, the early dates are subtracted from the late dates to calculate Total Float. In Figure 2 below, the early date of April 15 has been subtracted from the late date of April 10 resulting in 5 days of Negative Total Float.
This indicates that we have exceeded an important date along the critical path.
How is it possible to have a late date earlier than an early date?
In a typical Gantt chart schedule, shown below in Figure 3, there are three activities (A, B and C). Total Float is represented by the yellow boxes on each of the activities. They have float because there is a constraint on Activity C.
The ‘Finish on Constraint’ date is set later than the currently scheduled finish date of Activity C, therefore, some float exists for Activity C and its’ predecessors.
What causes this?
The project starts and the preceding activities begin to slip and the float starts to get used up. Activities A through to C become critical as their float drops to zero, as shown in Figure 4.
The project continues to slip and the Activities become supercritical with a Negative Float value. The ‘Finish on Constraint’ on Activity C has been exceeded, as shown in Figure 5 below.
The calculated early dates are now calculating a date later than the ‘Finish on Constraint’ date. The Constraint has locked the late finish date so a ‘Negative Float’ appears in the schedule, as shown in Figure 6. This means there are now zero days to complete the work.
If there is a Negative Float on the schedule, it typically indicates that a constraint date has been exceeded. If the constraint has been appropriately applied, for example, a ‘Contractual Delivery Date’ for First Draft’ as shown in Figure 7, the project will now miss that date.
Negative Float is an early warning sign, that you will fail to meet an important date in the schedule.
From a scheduling best practice perspective, you mustn’t have Negative Float appearing in your primary baseline. As this would indicate that you are planning to miss an important deadline in your schedule before work has begun.
You would fundamentally be planning to fail and your schedule won’t reflect a realistic approach to your project. Negative Float should only occur in a schedule after work has started and progress is being reported.
Summary
If you see Negative Float on your schedule it typically indicates that a constraint date has been exceeded. If you are using constraints in your schedule, particularly hard constraints, you should check for Negative Float before baselining your plan.
We’ve produced a short video, ‘Negative Float Explained’ where you can see the effects of this in a schedule. It shows tips on following good scheduling guidelines whilst working with Negative Float.
You can also see other Oracle Primavera P6 videos by Ten Six here as well as subscribe to our YouTube channel to receive notifications of when we post a new video.
For more Primavera P6 articles click here.