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Float at WBS Level / How Float is Calculated at WBS Level

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jean-michel albert
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How in all of the God's names is the 'Total Float' value shown at the WBS level calculated ?

For some WBS levels, the float appears to be the lowest (most critical) float value of the activities contained underthat WBS level.

However for other WBS levels, the float mayto be the highest (least critical) float value of the activities contained underthat WBS level , or even something in between.

I can not understand how the float value at the WBS level is calculated - can anyone help undersand this??????

For background, I am trying to share a 'rolled up' part of a program with my team, showing only WBS levels rather than hundreds of underlying activities. But they also need to see the lowest / most critcal float values for a WBS.

Thanks !


Zoltan Palffy
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agree dont use it just like Rocky 

Stephen Devaux
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Interesting discussion. And I agree that TF computation at a summary level is usually useless and sometimes misleading.

However, critical path drag computation at a summary level, especially on large programmes, can be very useful. It can show the amount of time that a project (within a programme), component, department, subcontractor, team or discipline/skill is adding to the project duration. This can then be shown to the reponsible group and it can drill down within the schedule of its own tasks to compute detail level drag and compress the drag its adding to the programme.

Additionally, if drag cost is computed, a suitable incentive/award can be offered to the trageted organization to reduce/erase the drag.

I have done this for departments within a corporate project, and see no reason why it wouldn't work within a large construction project. (Of course, I did my drag computations "by hand"--exactly how the s/w does the summary level drag computation may be a factor.)

Fraternally in project management,

Steve the Bajan 

Rafael Davila
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Do not forget calendars are assigned to activities as well as resources; they need not be equal but compatible/consistent.  In case of complex resource assignments you might also need to pay attention to assignment floats.

Resource Critical Path


Multiple calendars make summary floats meaningless, keep the focus on the activities and resource assignments no matter your software.

Santosh Bhat
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I concur with Tom, don't use it. In fact turn off the Group totals altogether. Being able to summarise data is nice, but its precisely that a summary. That other software tools let you link into summaries is a big no-no im my opinion. Why? Think of this way, if a summary is equal to 6, what are its consituents? 1+5? 4+2? 3+3? 

But to specifically answer your question, here's what the Oracle Help files say:

How data is summarized

Float: You can base total float of the summarized data on the start dates, finish dates, or most critical dates. Set this option in the Compute Total Float As field on the General tab of the Schedule Options dialog box (choose Tools, Schedule, Options). If you base float on start dates, and all activities are assigned to the same calendar, total float is the difference between the earliest late start and the earliest early start dates in workperiods. For finish dates, P6 Professional uses the latest late finish and the latest early finish dates to calculate total float. The most critical float is the lowest total float encountered in the detailed activities from each summary group.

If all activities are not assigned to the same calendar, P6 Professional defines total float based on the default global calendar.

So I'm guessing you might achieve what you want by changing the option to the "Most Critical", but be mindful of what this will do at the detailed activity level also.

jean-michel albert
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Apologies - I am using P6.

I've learned the WBS Level Float appears to be linked to the final activity to finsh under the WBS in question - therefore not a reliable indicator of the criticality of that WBS level (because, for example, an earlier activity could be far more critical than the last activity under any given WBS).

So here is my follow up question - can you change the way a WBS summarises float for the underlying activities, so that instead of showing float for the 'last' activity, it shows float for the 'most critical' activity under that WBS ???
For example, perhaps a 'User Preference' , 'Admin Preference' or 'Global Change' of some description ???

Tom Boyle
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jean-michel albert,

You don't say what software you are using....  If I'm remembering right, in both P6 and MSP, the earliest-early dates and latest-late dates roll up to the WBS or summary, from which the float is calculated.  If you have zero concurrency and zero gaps in your WBS  (i.e. pure waterfall), then the summary will be as expected.  This is rare.  Overall - TF is useless for summary activities.  Don't use it.  Good luck, tom