Question

What does it mean when your schedule has negative float? If you are the Project Manager or lead scheduler, what are your options to deal with negative float?

Answer 1

Negative Float: Negative Float is the amount of time beyong a project's scheduled completion that a task within the project requires.

Total Float is the amount of time a task within a project can be delayed without endangering the project dead.

1. Negative Float because of Imposed Constraint on the Project Finish Date:

Consider the project as our sample and pay attention to total float and early finish dates of this project:

On project , “Must Finish By” imposed date of our project is same as our scheduled finish date: 15-May-2018

Now assume that our project requirements have been changed and we need to finish our project plan on 15-Apr-2018, instead of 15-May-18.

After re-scheduling the project with this newly imposed date, notice what happens to the “Total Float” column  will have negative Total Floats. Essentially, this negative float is the amount of time that must be saved to bring the project to completion on time.

Option to deal with this Negative Float:

Well, we’re supposed to follow the project requirements, so we need to compress our project schedule.

The most common methods that can be used to shorten the project duration while maintaining the project scope are either “fast tracking” or “crashing” techniques.

In fast tracking, activities on the critical longest path that would have been performed sequentially using the original schedule, are assessed for the possibility of working in parallel. In using fast tracking method, if we cannot save enough time on the schedule, we can apply the crashing technique.

In the crashing method, resources are added to the activities on the critical path and the duration of activities are reduced as a result.

Going back to our sample project, by using either crashing or fast tracking techniques, the project finish date would go back to 11-Mar-2018 and all the negative total floats would be resolved.

2. Negative Float because of Imposed Constraint on an Activity or Milestone:

Now assume that we have another requirement on our initial schedule, which asks us to award the contract no later than 01-May-2017. Consequently, we need to define a constraint on “Award Contract” activity to represent this new requirement.

By imposing “Finish on or Before” constraint, the total float of this activity will turn negative. The reason is that adding this new date constraint has overruled the pre-defined relationship of this activity.

Solution:

The defined relationships of related activities need to be evaluated and modified so that the finish date of “Award Contract” activity goes at least on 01-May-2017 and consequently, the associated negative floats are resolved.

3. Negative Float on Summary Activities:

This is usually for who are scheduling in the primvera scheduling software duw to settings issue in this software we way arise this type of negative float.

Although it’s rare, there might be a possibility to have negative floats for “level of effort” or “WBS Summary” activities. The main reason is due to changing a setting in primavera p6 schedule option tab.

If you use level of effort or WBS summary activities in your schedule always make sure that the schedule calculates “Total Float” as “Finish Float=Late Finish-Early Finish”. Otherwise, you might encounter negative float for these kinds of activities.

In summary, you learned three main reasons why negative floats exist in a schedule and strategies to remove them:

• Negative Float because of Imposed Constraint on the Project Finish Date
  Solution: Remove any finish date constraints and compress the schedule by Fast Tracking or Crashing methods
• Negative Float because of Imposed Constraint on an Activity or Milestone
  Solution: Evaluate and modify the conflicts between constraint and relationships. Remove or change any “Hard constraints” that prevent the logical float calculation from flowing.
• Negative Float on Summary Activities
  Solution: Check the schedule options and to compute “Total Float” as “Finish Float=Late Finish-Early Finish”.