PMEducation
The VALUE of FLOATS
In creating our CPM diagram we calculated the Total Float for each Activity. Total Float is a measure of how tightly the Activity is tied to the finish date.The calculation for each Activity is TF=LF-EF. We can now put TF onto the Activity list as follows:
from: HOW TO MAKE A CPM DIAGRAM. Example #1
ABBREVIATED ACTIVITY LIST for making a CPM Diagram
Work Package Activity Predecessor Successor Duration (days) Total Float Free Float
Start up 1 Start 2 0 0 0
Start up 2 1 4 3 0 0
Start up 4 2 7,5,9 1 0 0
A 7 4 8 6 3 0
A 8 7 12 1 3 3
B 5 4 6 2 7 0
B 6 5 12 1 7 7
C 9 4 10 8 0 0
C 10 9 11 1 0 0
C 11 10 12 1 0 0
Finish off 12 8,6,11 13 2 0 0
Finish off 13 12 Finish 5 0 0
Also shown on the above table is Free Float. Free Float is a measure of how tightly the Activity is tied to the next Activity, its Successor. Let's look at this a bit closer.
Total Float (TF): The amount of time that an Activity can be delayed, from its Earliest Start, without delaying the Project Finish, or violating a schedule constraint
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TF=LF-EF
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Every Activity on a branch path has identical Total Float
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The Total Float for a branch path an be called the path slack for that branch
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For every Activity on the Critical path TF=0
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A positive number for TF indicates how far away that Activity is from the Critical path, and from delaying the project finish.
Free Float (FF): The amount of time that an Activity can be delayed, from its Earliest Start, without delaying the Earliest Start of any successor, or violating a schedule constraint.
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FF=ES next - EF
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The equation means the FF for an Activity is the ES for the next Activity minus the EF for the Activity in question.
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Activities on a branch path can have different Free Floats
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Free Float will be zero if its immediate successor can start as soon as the Activity is finished
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For every Activity on the Critical path FF=0
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For Activities that intersect the Critical path, their FF=TF
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A positive number for FF indicates how far away that Activity is from delaying the earliest Start of its successor.
These videos provide a nice summary for the topic of Floats.
They will load faster in Firefox than in Internet Explorer.
By now you realize you cannot determine Float until you make a CPM Diagram. Then you can list the Floats on the Activity list. Sometimes Total Float and Free Float are extracted from the Activity list to make a Float Table. Especially in large complicated projects, referring to a Float Table is a quick way to recognize how a delayed Activity might impact the next Activity and the project finish date (if at all). A Float Table is a handy tool for quickly identifying risks and implications from possible/proposed changes to the schedule, without having to re-calculate the whole CPD.
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