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Free Float Value in a Schedule

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Sebastian DiBlasi
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I have a schedule that the contractor submitted with 1,000 activities. Said schedule has approximately 40% of the activities with Total Float greater than 100 days. All of those activities have Free Float of zero. To make this even more fun, there is only one activity on the critical path when using TF less than 1 and 50 activities when choosing Longest Path.

My question is: What is the significance of Free Float and should it be evaluated as a better understanding of the project critical path or project longest path?

Replies

Anoon Iimos
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Joined: 22 Sep 2006
Posts: 1422

Rafael,

Did you know (for any chance) if "Resource Leveling" was considered when "CPM" was invented?

If "Critical Path" (and floats) was first derived from schedules that consider only Tasks, Logics and Assumed Durations; without considering resources by any chance, would this make the calculations valid or accurate?

cheers!

Rafael Davila
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Joined: 1 Mar 2004
Posts: 5228

Sebastian,

Did the contractor submitted a paper copy of the schedule so you can compare the schedule as seen within his database versus the imported file, this can create issues with resources and calendars that can be pointed out to be because of the import if both schedules do not match. This is an issue particular of P6, should not be an issue with most other software.

http://xenasoftware.wordpress.com/tag/importing-xer-files/

http://executiveportlets.com/xerman.html

The following link can provide some other clues as to the issues you are having with your particular software especially if you are using a Primavera Product.

http://www.alphacorporation.com/49R-06.pdf

Anoon,

I agree with you, even if your software is capable of displaying the correct values of float these are confusing especially when under resource leveling several activities might be delayed individually and have no effect on the project duration while if you delay a group it might then have an effect on the schedule finish date. Yes they are slippery but more if the numbers are wrong from the very beginning.

To make the problem even more dangerous on this side of the Atlantic, still some delay protocols like the AACE 29R-03 (http://www.virginiadot.org/business/resources/const/AACE_Recommended_Practice_Forensic_Schedule_Impact_Analysis-29R-03.pdf) makes continuous reference to float values instead of just using the software to show cause and effect which shall be the same, no matter if your software can or cannot display correct resource leveled float values. To make it even worse they embeded the concept of Longest Path into the concept of Critical Path that includes activities that are not critical as they still have float, by definition is wrong as the added activities still have some float and can still be delayed without impacting the project finish. Still the meaning of float shall be the potential delay that does not postpone project finish or due dates.

What is even more of a joke is the fact that in page 9 of the AACE 49R-06 Recommended practice for identifying the Critical Path, see http://www.alphacorporation.com/49R-06.pdf the authors make reference to Primavera Software and say "This RP assumes that the algorithms used in each of these implementations are identical and give identical results." Seems like even the AACE so called "experts" are unable to validate the correctness of Primavera products when a single wrong result would be enough to prove its unreliability. Making reference to a product you cannot validate is irresponsible.

If because of different calendars or resource leveling the critical path is discontinuous then let it be, just keep the correct mathematical meaning of float. Saying critical path can be this or another thing just bring confusion and is wrong. It seems like the implementation of longest path in P6 is not the same as that in P3 and have created even more confusion, or shall I say more fun.

By the same token negative float hides schedule logic to the extent the forensic analyst and many schedulers disable the constraints that create negative float. Still the AACE 29R-03 requires to consider negative float and this results in very cumbersome exceptions to be applied, for example.

under one of the many delays methodology,

1. Excusable & Compensable Delay (ECD)
The difference between the as-built completion date and the collapsed as-built completion date resulting from the extraction of all owner-caused delays is the total ECD. If the owner has paid the contractor specifically to accelerate, then any negative delay durations (delay mitigation) resulting from the owner-paid acceleration should be credited to the owner against the total ECD to avoid double payment to the contractor for acceleration. Where the quantification of the duration of the specific paid mitigation is not reasonably feasible, the credit adjustment may be accomplished by crediting the monetary value of the acceleration payment against the monetary value of the ECD.

or under other delay methodology,

1. Excusable & Compensable Delay (ECD)
The difference between the as-built completion date and the collapsed as-built completion date resulting from the extraction of all owner-caused delays is the total ECD for each modeled time period. If the owner has paid the contractor specifically to accelerate, then any negative delay durations (delay mitigation) resulting from the owner-paid acceleration should be credited to the owner against the total ECD to avoid double payment to the contractor for acceleration. Where the quantification of the duration of the specific paid mitigation is not reasonably feasible, the credit adjustment may be accomplished by crediting the monetary value of the acceleration payment against the monetary value of the ECD.

... and the workaround to deal with negative float are endless, what a mess !!!

http://www.mosaicprojects.com.au/Resources_Papers_043.html 

In my claims I keep it simple and avoid any reference to the AACE recommended practices as even the most basic, the 52R-06 (http://www.virginiadot.org/business/resources/const/AACE_Recommended_Practice_Time_Impact_Analysis-52R-06.pdf) wrongfully advocate for the use of a fragnet without mentioning other possible routes as if non acceptable, when in many occasions the most adequate method calls for the use of calendar exceptions. While in such cases it might be adequate to display as an activity the delay non-work days the procedure fails to mention at times it can best be modeled adding calendar exceptions and therefore is misleading, is in error. Such a document on the hands of an inexperienced inspector can become an issue and provides no help to the newcomer.

Best regards,

Rafael

PS. Don't believe everything you read just because it is on print, there is a lot of B.S. on print out there.

Anoon Iimos
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Joined: 22 Sep 2006
Posts: 1422

The significance will depend on your schedule. Is your schedule "Date Driven" (without resource loading) or "Resource Driven" (fully loaded)?

For me, when using software, float calculations becomes confusing. You might get accurate values when running the schedule, but after levelling resources, you might get different values as well.

So for thousand activities (or more), I might suggest that you forget about floats.

Please allow me to add another question: What is the most important? Activity Driven Floats (Tasks, Logics, Durations - Without Resource Loading); or Resource Driven Floats or Resource Floats?