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Can a float be negative? how?
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Can a float be negative? how?
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You can think of float as the gap between EF and LF, or ES and LS. Also Lag and Lead is float. So in this cse, lead time could be considered 'negative' float.
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1 reply by Thomas Boyle
May 23, 2018 4:14 PM
Thomas Boyle
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Andrew, Lag and Lead [what most people call negative lag] are inputs to a PDM model; Floats are computed outputs. To say, “Lag and Lead is float,” simply confuses the issue. The statement, “lead time could be considered ‘negative’ float,” is opposite to my understanding. If anything, a lead could be considered a “negative float eraser.” In a negative float (i.e. behind schedule) regime, imposing leads is one of the first methods adopted by schedule tricksters to erase the negative float and “recover” the schedule – without requiring more explicit changes to the management plan. Since such techniques can be easily used to obscure the true schedule status while increasing schedule risk, leads are explicitly prohibited in many/most project scheduling specifications.
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In addition to Andrews response: On the critical path float is zero. In case float is negative the completion of lconnected tasks is behind schedule (completed later than planned) and the schedule needs corrective action (schedule compression).
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1 reply by Thomas Boyle
May 23, 2018 4:15 PM
Thomas Boyle
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Peter, “On the critical path float is zero” is a true statement in some classical situations and perhaps for those software packages that do not support negative total float. The more accepted definition is “On the critical path Total Float is the lowest value,” subject to numerous caveats about multiple constraints, calendars, and resource leveling that make any float-based definition invalid in many cases.
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I totally agree with my fellow colleagues. Negative Float means that activity is or will be behind schedule.
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Yes float can be negative.


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Great way to quickly check for schedule integrity. Negative float indicates some constraint has been breached or there is some other issue with activity logic.
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1 reply by Thomas Boyle
May 23, 2018 4:16 PM
Thomas Boyle
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John, I’ve not seen a correlation between negative float and poor “schedule integrity,” i.e. “some other issue with activity logic.” In my experience, high-integrity schedules lead to more accurate forecasts – and more encounters with negative float during project execution. Of course, negative float in a baseline submittal is a different case altogether….
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Missed this on the first go round and figured to start with the original questions:
Can a float be negative?
Free float, Independent float, Junior Float – No
Start Float, Finish Float, Total Float – Yes, depending on your software.

How?
1. Late Constraints:
a. Application of a Late Constraint or Deadline is allowed to accelerate the Late Finish (LF) of an activity compared to the LF that would be computed based solely on the forward and backward passes.
b. The backward pass transmits this acceleration of the Late Dates to all predecessors of the constrained activity.
c. Consequently, the Late Dates of the constrained activity and all of its predecessors may precede the corresponding Early Dates (from the forward pass). This results in negative Total Float.
Note: Computation of Negative Total Float is not universally agreed. Some scheduling software does not permit condition (a) above, so negative Total Float is not computed. Other software permits the condition but reports the resulting negative Total Float as an error. The dominant general project scheduling tools in North America (Oracle P6 and MS Project) both compute negative Total Float as shown.
2. Calendar Offsets:
In some software, the calendar of a Finish Milestone is effectively imposed on the Late Finish of its predecessors. This can create negative Total Float (Late Finish less than Early Finish) if the calendars of the milestone and its predecessors are different.
3. Other Issues:
Progress updating involving non-FS relationships, leads, lags, suspend/resume, and out-of-sequence execution can occasionally (rarely) cause negative total float, depending on detailed software algorithms and settings.

What does it Mean?
In most cases, the existence of negative total float in a schedule simply says that one or more constraints/deadlines will not be met. I.e. the project is behind schedule. For an individual activity, negative total float means that the activity is scheduled too late and must be accelerated by TF x (-1) to avoid violating a constraint and thereby contributing to the behind-schedule status of the project. Identifying which activities violate which constraints – and more importantly which activities are driving the violated constraints – often requires further study.

I do agree with the prior responses with a few quibbles noted.
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Aug 09, 2017 6:46 AM
Replying to Andrew Craig
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You can think of float as the gap between EF and LF, or ES and LS. Also Lag and Lead is float. So in this cse, lead time could be considered 'negative' float.
Andrew, Lag and Lead [what most people call negative lag] are inputs to a PDM model; Floats are computed outputs. To say, “Lag and Lead is float,” simply confuses the issue. The statement, “lead time could be considered ‘negative’ float,” is opposite to my understanding. If anything, a lead could be considered a “negative float eraser.” In a negative float (i.e. behind schedule) regime, imposing leads is one of the first methods adopted by schedule tricksters to erase the negative float and “recover” the schedule – without requiring more explicit changes to the management plan. Since such techniques can be easily used to obscure the true schedule status while increasing schedule risk, leads are explicitly prohibited in many/most project scheduling specifications.
Network:0



Aug 09, 2017 7:02 AM
Replying to Peter Ambrosy
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In addition to Andrews response: On the critical path float is zero. In case float is negative the completion of lconnected tasks is behind schedule (completed later than planned) and the schedule needs corrective action (schedule compression).
Peter, “On the critical path float is zero” is a true statement in some classical situations and perhaps for those software packages that do not support negative total float. The more accepted definition is “On the critical path Total Float is the lowest value,” subject to numerous caveats about multiple constraints, calendars, and resource leveling that make any float-based definition invalid in many cases.
Network:0



May 22, 2018 5:52 PM
Replying to John Aucoin
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Great way to quickly check for schedule integrity. Negative float indicates some constraint has been breached or there is some other issue with activity logic.
John, I’ve not seen a correlation between negative float and poor “schedule integrity,” i.e. “some other issue with activity logic.” In my experience, high-integrity schedules lead to more accurate forecasts – and more encounters with negative float during project execution. Of course, negative float in a baseline submittal is a different case altogether….
Network:891



If it won't work on paper, it won't work in the real world. Negative float in a schedule is a bad sign if a commitment is going to be made based on the milestone dates in that schedule. However, as Thomas indicates, once actual activity begins against the baseline, you could find yourself in a negative float situation...

Kiron
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