Introduction
Negative Total Float values are commonly interpreted as an indication that a Project is not meeting its completion requirements. This is often due to the presence of an imposed constraint such as “Finish-On-or-Before” or “Must Finish By” on a task, milestone or the Project in the CPM network. When Negative Total Float values occur, it typically signals the need for corrective action(s) to recover from any resulting delays or impacts.
Negative Total Float Values Without Constraints
While constraints are the usual cause of negative Total Float values, it is possible for negative Total Float values to occur even in their absence. This scenario can arise from the way calendars are assigned to different tasks and milestones within a schedule.
Illustrative Example
Consider the Project’s “Zum_NTFL” CPM network shown in Figure 1 above. For clarity, the Early Start, Early Finish, Late Finish and Calendar assignment columns are displayed. In addition, the Date Format is expanded to show the Time with AM/PM. Finally, the Data Date is set to Monday, June 1 at 8:00 AM.
- Activity A1000 – “Milestone A” is a Start Milestone and is assigned to the PC – Standard 5-Day Work Week Calendar.
- Activity A1010 – “Task B” is assigned to the PC – Standard 5-Day Work Week calendar and has an Original Duration of 2 Days, resulting in an Early Finish of Tuesday, June 2 at 5:00PM.
- Activity – A1020 – “Task C” is assigned to the PC – Standard 7-Day Work Week calendar and has an Original Duration of 4 Days, resulting in an Early Finish of Saturday, June 6 at 5:00PM.
- Activity A1030 – “Milestone D” is a Finish Milestone and is assigned to the PC – Standard 5-Day Work Week calendar.
- The 7-Day and 5-Day Work Week calendars are configured so that the Detailed work hours/day setting include 8 Work hours per day from 08:00 thru 12:00 and from 01:00 thru 05:00.
Since “Milestone D” is assigned to the PC – Standard 5-Day Work Week calendar, the calendar’s setting does not allow for a Saturday finish. As a result, the Early Finish for “Milestone D” is Monday, June 8 at 08:00 AM. Note that the time is 08:00 AM (instead of 5:00PM) because that is the earliest available date/time in the PC – Standard 5-Day Work Week calendar.
During the Backward Pass, the Late Finish for “Milestone D” begins at latest available Finish Date based on the PC – Standard 5-Day Work Week calendar which is Friday, June 5 at 5:00 PM. Keep in mind that with respect to the PC – Standard 5-Day Work Week calendar, 08-Jun-26 08:00 AM and 05-Jun-26 05:00 PM are effectively the same point in time.
Since “Task C” is a direct predecessor to “Milestone D”, “Task C”’s Late Finish is also calculated to Friday, June 5 at 5:00PM. However, that date is 1 day earlier than its Early Finish of Saturday, June 6 at 5:00PM. Therefore, the Total Float value for “Task C” is -1 Day and the Backward Pass propagates this negative value through the remaining CPM network.
Accumulation of “False Negative” Total Float values
In complex CPM networks with numerous activities and multiple calendars, this effect can compound, resulting in what can be classified as “False Negative” Total Float values. These values do not necessarily reflect actual schedule problems but are rather a result of calendar misalignment.
This anomaly is also noted in the “Warnings” section of the P6 Schedule log text output file when a Finish Milestone has a different calendar than its predecessor activity (See Figure 2 below).
Solution
To resolve the issue illustrated above, one simple solution is to reassign “Milestone D” to the same 7-Day Work Week calendar used for “Task C”. Once this adjustment is made, recalculating the schedule will eliminate the negative Total Float value, restoring accurate Total Float values throughout the CPM network.
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