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Calculating Float in P6 with multiple calendars

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Ed Arnold
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When using multiple calendars for different activities within a schedule (24hr calendars, standard 5 day 9-5 calendars etc) I get an extrenely high value in the total float column for activities with a 24hr calendar. The total float figure is longer than the remaining duration of the project. All activities are linked and I have put a must finish by constraint on the final activity - any ideas???

Replies

Khuong Do
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Hi,

I also find a wrong critical path in Primavera when we use multicalendar.

I write about it in this article https://doduykhuong.wordpress.com/2016/02/17/critical-path-by-total-floa...

Rafael Davila
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Joined: 1 Mar 2004
Posts: 4780

Arnold,

About the late bars P6 would display wrong when float calculation is wrong maybe you can calculate them manually and instead of using permanent ink you can use erasable ink on your screen so if you get them wrong you still can correct them.

For the S curve suggest using a second monitor, here you can use permanent ink to adjust the late S Curve, it will also solve the issue of distorted late S curve when allowing for negative float. In this case late S curve will be based on impossible late dates and you will be modeling the impossible, under resource leveling this aberration is greater so you will be killing two birds on a single shot.

By the way if negative float makes your early and late S curves intersect at some point before project end date better don't fix it, it is funny, it drives crazy many people and you will have the opportunity to say "it is just a mathematical computation" and your wrong modeling will be justified.

If someone tells you it is wrong, just reply them "you are making too much issue of a simple BUG" that you will fix in a future version of your reports.

Anoon,

All I know is that, at first, you define your activities and then schedule it. Second, is to allocate your resources.

Again you got it right and hope you can understand that while disclosing horrendous errors in some software even when is for a serious cause, for the benefit of the PP community, there is nothing wrong with having some fun as long as with the fun part you disclose true arguments. 

Best regards,

Rafael

Anoon Iimos
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Posts: 1416

Hi Rafael,

I'm not sure if what do you mean by "soft links" which can be easily changed. What I mean is "concrete link" that is not technically changeable, and you call this as "resource constraint"? Then "Finish-to-Start" relationship is a resource constraint?

All I know is that, at first, you define your activities and then schedule it. Second, is to allocate your resources.

In my interpretation, you are saying that defining activities means constraining resources.

Please explain further.

Best regards,

Edgar

 

Rafael Davila
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Anoon,

You got it 100% correct, I schedule both, activities and resources. I mostly use logic relationships and let the software figure out the resource dependencies as pre-defining resource dependencies will not allow the software to determine optimal or near optimal resource allocation. I emphasize the resource leveling and resource loading as without resources you cannot build it, usually resources are limited and represent very relevant constraint you cannot overlook.

For your knowledge I am not against so called "soft links" when they represent a human decision not necessarily based on technologies. I am against their use to artificially solve resource constraints.  The use of soft links based on human decision is common in linear jobs such as when you decided to build a road from A to B when you had other options such as to build from B to A, from both sides, etc. But you selected an option and create dependencies that support your decision.

In any case these links shall be easy to change, so it is necessary to be able to filter them. Spider as well as Asta PP provide for you to label these and filter them. I do not believe MSP nor Primavera allow you to do so.

Same as to many,float at times confuses me, but displaying wrong values of float makes it even more difficult. It is annoying reading a wrong total or free float value when looking for ways to get on track your job you make use of it just to find out the model does not behaves as expected and you are in a meeting with the whole planning crew. Spider gives me the correct values of float for the displayed solution and therefore no longer I get these misleading values, by getting rid of these misleading values it is "less difficult" to get the job on track.

Best regards,

Rafael

Anoon Iimos
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Hi Rafael,

I supposed there is a misunderstanding between "Scheduling" and "Resource Levelling" when calculating floats.

I believe that your emphasis is on resource levelling (for not considering task relationships).

What do you schedule by the way? Activities or Resources? or Both? If both, then I supposed that you must define some concrete relationships between tasks (I'm not sure though, you are the expert).

Kindly explain.

cheers!

Edgar 

Rafael Davila
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Arnold,

It is a secret everyone knows, P6 float calculations are wrong, so wrong even a resource leveled two activities schedule is enough to prove it. I believe it got to be frustrating to serious professionals who have no option than to deal with it.

In my 4 activities sample job, to my surprise P6 got the total float values right but not the free float values, weird, very weird, you have no total float but free float, this is good understanding of CPM math.

FF vs TF

Yes I know, GIGO, garbage in garbage out but with P6 it is always garbage out no matter if the input is not garbage, you never know if the displayed values of total and free float are correct.

You have the option to assume float and late dates are irrelevant and cross with permanent ink your computer screen where the float and late date values are displayed, about late bars I have not figured out a permanent ink solution.

Trash it.

Paul,

Does Asta yields correct float values after resource leveling? I would not be happy with software that yields correct float only under specific conditions. What if you run a job consisting only of four activities with a duration of five work days all requiring Resource A of which only one is available and none of the activities are linked to each other, will it display correct float values?

I do not understand what you mean with results you are happy with, do you mean you are not sure about their correctness? I believe that by delaying an activity a duration less than displayed float until it reaches the displayed value then the effect on the successors if free float and on the job duration if total float then you can validate at least the values under investigation. Will not tell you about all possible scenarios but finding a single wrong value is enough to demonstrate the calculations cannot be trusted.

Dieter,

"Calculation of total float is an excellent mean to confuse planners and managers." Now imagine if the calculation is wrong! Still even when displayed value is correct I agree with you, float is slippery. In my four activities sample job if you delay the first activity by one day all activities are delayed by one day, but if you delay the first activity by 5 days, the second activity will take its place and the job will not be delayed, the solution set is not a single value nor continuous and this creates confusion. Depending on the resource usage and the network conditions, there could be alternative start and finish times for certain activities in the same schedule without delaying the project completion time.

Even if your software can display true values for all solution sets no protocol shall make reference to it as a mean to solve delay claim issues, cause and effect shall prevail. The common statement that float belongs to whoever uses it first shall be substituted with something better. A good protocol shall warn about the traps of float interpretation.

Regards,

Rafael

Paul Harris
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It has been my experiance that P6 does not calculate float correctly when multiple calendars are used, I have produced a schedule without any constraints and resulted in a -36day total float. It does not handle change of calendars in certain combinations and will give incorrect or difficult to understand results.

After several emails to Oracle the response was that it was acknowledged this as a bug and would be put on the list of fixes.

When I come accross these issues I run the scenario up in P3, SureTrak, Asta PowerPorject, Microosft Project and P6 and Asta is the only product that give a result that I am happy with.

Paul E Harris

Eastwood Harris Pty Ltd, Melbourne, Australia

Planning and Scheduling Training Manual & Book Publishers, Consulting and Training

www.eh.com.au

www.eastwoodharris.com

www.primavera.com.au- a Primavera scheduling software resources site

Dieter Wambach
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Ed
Calculation of total float is an excellent mean to confuse planners and managers. In a different thread I wrote already that recently all main German EPC contractors had a meeting with Primavera. This was one of the items. We asked Primavera to add an additional field where the float is calculated based on the default project calendar, to e.g. being able to display a critical (longest) path with the same total float. Float is calculated for each activity based on its calendar, for summary lines based on the default calendar of the database (not of the project!). Currently we have to hide the float if different calendars are used.
Why: All calculations are done on basis: hours. Later depending on your entries (e.g. 1 day = 8 hours,...) it will be displayed as days etc. For this calculation a day with 24h represents 3 days (24 / 8 = 3)!
--> In my opinion no board member will understand such a report. Therefore maybe it’s better to hide.
Regards
Dieter
Raviraj Bhedase
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If all activities are not assigned to the same calendar, the Project Management module defines total float based on the default global calendar.