Negative Total Float typically means that the Project is not meeting completion requirements (usually represented by a Constraint) and therefore efforts must be initiated in order to recover from the realized impacts.
However, Negative Total Float can occur even if the Project has no constraints whatsoever. In the illustration above, the Data Date is Monday, June 6 and Task C is assigned to a 7 Day Calendar with an Original Duration of 3 Days. Finish Milestone D is assigned to a 5 Day Calendar. Therefore, the Early Finish for “C” is Saturday, June 11. Since “D” is assigned to a 5 Day Calendar, it cannot end on Saturday, therefore, P6 calculates the Early Start as Monday, June 13 at 08:00 AM.
On the Backward Pass, the Late Finish for “D” and “C” are calculated as Friday, June 10 at 05:00 PM but that is 1 Day earlier than the Early Finish for “C”. Therefore the calculated Total Float for “C” is -1. The Backward Pass propagates that result back through the CPM network to “A”.
In a CPM network with many activities, and multiple calendars, it’s possible that this effect could accumulate resulting in “False Negative” Total Float values.
The solution in this illustration is to reassign “D” to the 7 Day Calendar. Then recalculate the schedule.
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