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
Every Activity on a branch path has identical Total Float
The Total Float for a branch path an be called the path slack for that branch
For every Activity on the Critical path TF=0
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.
FF=ES next - EF
The equation means the FF for an Activity is the ES for the next Activity minus the EF for the Activity in question.
Activities on a branch path can have different Free Floats
Free Float will be zero if its immediate successor can start as soon as the Activity is finished
For every Activity on the Critical path FF=0
For Activities that intersect the Critical path, their FF=TF
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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