What are the Causes of Negative Float in Oracle Primavera P6

There are not very many definitions of Negative Float in documents that have a list of scheduling definitions and AACE Recommended Practice 10S-90 Cost Engineering Terminology defines it in as:

1. The amount of time by which the early date of and activity exceeds its late date. It is how far it is behind an activity is behind its planned early start/finish date.
2. Time by which the duration of an activity has to be reduced in order to permit a limiting imposed date to be achieved. (June 2007)

Scheduling software calculates Negative Float when the Late Dates are earlier that the Early Dates and is calculated by the subtraction of the Late Finish minus the Early Finish calculated over the calendar assigned to the Activity.

The issue of why a schedule has Negative Float is as a result of the algorithms that the software used to calculate the Late Finish.

Activity Constraints

Setting a Finish Constraint such as a Finish On, Finish On or Before or Mandatory Finish before the calculated activity Finish Date will generate Negative Float.

The finish constraint is set two days before calculated activity finish date of A1020 and 2 days Negative Float has been calculated.

Project “Must Finish By” Date

Setting a Project Must Finish By date will generate Negative Float.

The Project Must Finish By date is set three days before calculated project finish date and 3 days Negative Float has been calculated.

The picture on the right shows a project where the user was not displaying the time and when they set the Finish On or Before constraint the software set the time to be midnight at the start of the day and thus the Finish of the activity is calculated at the evening before resulting in negative float.

When the time is displayed the issue is obvious and the user may now see that the constraint time is at midnight at the start of the day, thus generating Negative Float.

P6 Version 21 now shows the time in the Date Picker Box which helps to resolve this issue and this version seems less prone to setting constraint times at midnight at the start of the day.

Setting a Project Must Finish by Date without showing the time will also set the Project Must Finish Time at midnight, the picture on the right shows that P6 will select midnight at the start of the day when setting a Project Must Finish By date.

This again will create Negative Float when the Project Must Finish By date is set at the same date as the calculated finish date but at Midnight at the start of the day.

When a project has link to a project with successors activities that have a constraint the negative float will be reflected in the predecessor project’s activities.

When the project is opened on its own the Negative Float is still calculated.

But if you check the Tools, Schedule, Options, Ignore relationships to and from other projects, then relationships from other unopened projects are ignored.

When a project is exported and imported into another database the missing external relationships are replaced with External Early Start and External Late Finish constraints which will create negative float:

• The project on the right had the Negative Float Development Project exported and then imported into another database.
  • Before exporting;
  • After importing into another database, the External Constraints will create Negative Float;
  • The External Constraints may be deleted but if you check the Tools, Schedule, Options, Ignore relationships to and from other projects, then the External Constraints are also ignored which is not clear from the option title.

Setting Multiple Baselines

When you open multiple projects and set the baselines for all the projects then restoring these baseline projects will create ghost relationships and the restored projects and current projects may not calculate correctly and may create Negative Float in any of the projects. The text below is an extract from my P6 books explaining the issue and the fix.

The following process is NOT RECOMMENDED because issues covered next when restoring baselines set this way. The Baselines may be set for all the projects using the Maintain Baselines form (when multiple projects are open) and the Assign Baselines form. The picture on the right shows the process of setting multiple project baselines:

• Open the Maintain Baselines form by selecting Project, Maintain Baselines…:
  • Either all projects may be selected and a copy of all projects set as the Baselines at one time, or
  • Other current projects may be converted from the database one at a time.
• Select "Add" to open the Add New Baseline form and create the new baselines;
• Select Project, Assign Baselines… to open the Assign Baselines form and select one project at a time to assign the baselines.

Remember, a User Baseline set by one user will not be displayed when another user opens the project. The <Current Project> Baseline displays the Planned Dates from the current schedule and will be shown as a baseline.

Schedulers often wish to restore baselines to inspect or review the original schedule.

The process using Save a copy of the current project as a new baseline identified in the previous page results in one interesting issue when Baseline projects are restored. The software creates Ghost Relationships between the Current Schedules and Baseline schedules which must be avoided because there is a high risk that neither the Baseline nor Current projects will calculate correctly once Multiple Project Baselines are restored.

The example on the right explains what happens when three simple projects are baselined together:

• The three projects were opened together and baselined and restored;
• The baselines were unlinked and restored;
• When the current and baseline projects are opened together there are unwanted Ghost Relationships created by the software, without warning, between the Current and Baseline projects;
• When the current projects are opened on their own and activity durations shortened you will see that the schedule does not calculate correctly because of the Ghost Relationships.

Therefore, if you wish your baseline projects to maintain the relationships to other baselined projects only and not have Ghost Relationships created with the current projects when the multiple current projects with relationships amongst them are baselined, then you must:

• Open the Projects Window;
• Copy the multiple projects in this view Projects Window;
• Then set the baselines using the Convert another project to a new baseline of the current project option in the Maintain Baseline form;

Now if the baselined projects are restored there will be no Ghost Relationships created.

When an LOE activity has progress and the Schedule Options are the Default (see picture on the right).

There is no Negative Float on LOE activities (see picture on the right).

But if the Schedule, Options for Compute Total Float as bare are changed to Start Float = Late Start – Early Start (see picture on the right).

Then Negative Float is calculated on the LOE activity although the Late Start and Start are the same, but the Early Start is actually the Remaining Early Start and thus the Negative Float (see picture on the right).

There has been a longstanding issue with Finish Milestones with a predecessor that has a Lag and another calendar which often calculate Negative Float.

The issue can be resolved by changing the Finish Milestone to a Start Milestone.

When the Scheduling, Option, When scheduling progressed activities use is set to Actual Dates then often Negative Float is generated and also Circular Relationships may not be identified.

Please read my article Understanding the P6 Scheduling Options “When scheduling activities use, Retained Logic, Progress Override or Actual Dates” by clicking on this link:

Understanding the P6 Scheduling Options “When Scheduling Activities Use, Retained Logic, Progress Override or Actual Dates”