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The story behind the negative float in P6

The negative float is a very popular hot topic in the scheduling field and it’s generated when the late dates are less than the early dates and usually it’s accompanied with a delay in the project, and a lot of project management professionals think that the only reason for a negative float is because a constraint that has been used in the schedule. Although the first thing you look for when you see a negative float is the constraints that are applied to the schedule, but that not the only reason that generates the negative float in P6.

In this article I will go through all the possible scenarios that I have discovered throughout my experience that’s responsible for generating negative float.

1 – The usage of constraints

This is the most popular reason for generating a negative float, when we apply a constraint date which is the less than the early finish of the activities, the constraint date will replace the late finish of the activity and the negative float will be generated.

2 – The usage of MUST FINISH BY in the project settings

Sometimes we forget the must finish by in the project settings especially when we take copy from another project to work on it as a template, when the must finish by is applied it’s exactly as if we are using the Finish on constraint.

3 – External relationships

When we have an external relationship with other projects, that may lead to negative float, since the late dates are taken from the successor activities. That may happen when you open two projects at the same time especially if they are the same project but different revisions. Or when you are using the SDK to import the relationships and used the wrong predecessors or successors project ID.

To detect whether you have external relationship or not, either to check the Project ID column in the activities that have negative float (Start with the last activity if you sorted by finish date) or export the relationships using the SDK sheet.

4 – Using Level of Effort in a progressed schedule + Compute float settings using the Late start – Early start

If your setting for calculation the float is set as Late Start – Early start, you will experience a negative float for the Level of efforts that linked to progressed activities, the reason for that is because P6 actual dates override the late date for the LOE activities.

5 – Using multiple calendars

This is obviously a very old bug in P6 that hasn’t been fixed for ages now, if you used multiple different calendars with different hours in the schedule, that may generate a negative float, especially if there are milestones in the schedule, it seems that the late date of the milestone jump to the last working hour of the previous working day in the calendar.

 To fix this issue, find the milestones that have late date less than the early dates and total float equal zero, and dissolve these milestones. then hit F9

6 – Using Resources dependent activity type

Using the resource dependent activity type with a resource calendar different than the activity calendar may lead also to negative float, this is another bug that primavera needs to work on.

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