If you’ve ever opened a Primavera P6 schedule and wondered where all that negative float suddenly came from, then this article might help.
Firstly, negative float appears when an activity that has Late dates that are earlier than the Early dates. In other words, the critical path dates now exceed a hard constraint date on one or more of the activities and are reporting this fact with the negative float calculation. If you see negative float, something important is now scheduled after its delivery date.
Normally such activities are easy to find with a filter or two and a little column adding in the table. However, there is one constraint that doesn’t appear in the network but can create all manner of chaos if you don’t know it’s been set.
The Must Finish By Date
The Must Finish By date can be thought of as a project level constraint. Once the last activity in the schedule exceeds this date, negative float will appear.
This can be tricky because the Must Finish By date isn’t assigned to a specific activity. Instead, it’s a project constraint, so check the Projects window’s Date tab in the details area and make sure that the Must Finish By date is either not set to anything at all, or if utilized, is set to at least one day past the calculated project finish date.
Of course, if the project really must finish by a certain date and this field has been set in Primavera P6 accordingly, then normal schedule delay mitigation processes apply.
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