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Float Constraints

9 replies [Last post]
Thu, 2005-03-03 18:04
Bernard Ertl
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I understand where constraining the free float to zero (aka ALAP scheduling in most software that offer it) can be useful in planning some scenarios. However, I have never encountered a situation where constraining the total float to zero (forcing it to be critical) would be desired. Does anyone use this in practice? Can you provide an example where this is a useful construct?

What if a software allowed you to specify a "no greater than" total float constraint? Would that be useful?

Bernard Ertl
eTaskMaker Project Planning Software
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Replies

Fri, 2005-03-04 13:20
#1
Vladimir Liberzon
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Sorry, I did not understand.
An example - just in time work. You apply ALAP attribute to some activities if you want them to be executed "just in time".
Another example - you may want to optimize project cash flow moving expenses as late as possible without violating project finish date.
Vladimir
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Fri, 2005-03-04 13:13
#2
Bernard Ertl
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Vladimir, I am asking about the usefulness for using a planning construct (forcing tasks to be critical by moving their ES/EF to match their LS/LF) in developing a project schedule - not about analyzing total float.

Bernard Ertl
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Fri, 2005-03-04 12:36
#3
Vladimir Liberzon
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Bernard,
I don’t understand the difference between project management and project planner views.
One of the simple useful things - we usually suggest to check if activity total float is too large. It may mean that some dependencies are omitted.
Resource constrained total floats are similar to feeding buffers in Critical Chain project management theory.
It is easy to check which activities constitute resource critical path for any project phase - they belong to the phase and have the same total float as the phase as whole.
Vladimir
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Fri, 2005-03-04 11:58
#4
Bernard Ertl
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David, OK. I see that your method for forcing a critical path is to adjust the LS/LF instead of ES/EF. Sort of like an imposed, dynamic finish date constraint equal to the EF. I can see the usefulness for that.

What I have seen with some systems is the ability to set a float constraint that moves the ES/EF to the latest possible moment effectively delaying the task and all successors.

Vladimir, the question is aimed from the planner/scheduler perspective - not management’s. Is such a construct practical for modeling some real project scenario?

I’m thinking not. It (and the "not greater than" float filter I proposed) seems to me to be a feature that would be more useful for "gaming" the schedule than modeling any real logic. Although, I am interested to hear if someone has some project experience that could show otherwise.

Bernard Ertl
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Fri, 2005-03-04 10:55
#5
Vladimir Liberzon
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Bernard,
resource constrained total floats show feasible time reserves you have on any schedule activity. This information is used when you consider potential delays in connection with change requests, risk events, resource workload, etc. Please clarify what to explain.
Total floats calculated by most PM software do not consider resource constraints on the backward path and thus are useless if resources are limited. If you mean this kind of total floats then I agree that they cannot be utilised.
Vladimir
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Fri, 2005-03-04 10:27
#6
David Bordoli
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Bernard...

I hope I am not getting mixed up about what you are saying, if so I apologise and also if I am teaching you to suck eggs!

Imagine a project that has two or more buildings, each have to be handed over separately on different dates. The final activity in each string would be a handover milestone. I would mark that milestone ‘ASAP force critical’ (that is a Powerproject term but I am sure other software have similar features). This results in the network still being dynamic but the critical path being highlighted through the string.

Your filtering code sounds interesting but in general I am wary about filters. Too much experience in opening programmes that don’’ seem quite right only to discover some dodgey filter has been applied to manipulate the programme in some way.

Regards

David
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Fri, 2005-03-04 10:00
#7
Bernard Ertl
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I am sure that total float reserves are most useful in schedule management.Vladimir, does this statement imply that you have seen total float constraints employed in a schedule to model a real world situation? Could you elaborate?

David, if your software allowed you the ability to highlight critical logic paths through subsections of an overall schedule (say filtered by a sort code), would you still find a total float constraint useful? If I understood you correctly, you are changing the ES/EF for a section of the schedule just to highlight it. That seems dangerous to me from a risk perspective.

Bernard Ertl
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Fri, 2005-03-04 06:10
#8
David Bordoli
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Bernard

I use ’force critical’ where I have a project with a number of sectional completion dates so I can see a critical path for each section. Does that make sense?

I try not to use ’must complete by’ type restraints, as I don’t like the concept of negative float (that’s just a personal preference).

David
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Thu, 2005-03-03 23:57
#9
Vladimir Liberzon
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Bernard,
you described Spider Project approach to ALAP scheduling. I am sure that total float reserves are most useful in schedule management. And of course total floats should be calculated taking into consideration all project constraints including resource, financial and supply constraints and imposed dates.
But if a single activity has ALAP attribute it wll use only free float.
Vladimir
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