I recently had a discussion with a student who was trying to track down the reason for Negative Float on an activity and I decided to write this article as a check list for people who want to track down the cause of Negative Float in their program.
There are a number of reasons why a P6 program will display Negative Float, some are well known and expected, some less well known and some not expected.
The text below is a sumary of the causes but my full article may be found here:
The aim of the article is to list the reasons why a P6 program will display Negative Float under the following headings:
- Well Known and Expected,
- Less Well Known and
3 Negative Float Definition
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 the as:
- 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.
- 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 Lates 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.
You may wish to buy one of my P6 book by clicking on this link below to learn more about how P6 works:
4 Well Known and Expected
4.1 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.
4.2 Project “Must Finish By” Date
Setting a Project Must Finish By date will generate Negative Float:
5 Less Well Known
5.1 Setting a Constraint without Displaying the Time
When a user is 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.
5.2 Project “Must Finish By” Date Without Displaying the Time
Setting a Project Must Finish by Date without showing the time will also set the Project Must Finish Time at midnight. 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.
5.3 External Relationships
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:
5.4 External Constraints
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 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:
6 Unexpected Reasons for Negative Float
6.1 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.
6.2 Inprogress LOE Activity
When an LOE activity has progress and the Schedule Options are the Default there is no Negative Float on LOE activities. But if the Schedule, Options for Compute Total Float as bare are changed to Strat Float = Late Start – Early Start 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.
6.3 Finish Milestones and Lags
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.
6.4 Schedule Options and Circular Relationships
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: