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Support Center » Knowledgebase » PMP e-SIM Expert Comments » Float - Pre-test Monitoring and Controlling question #7
 Float - Pre-test Monitoring and Controlling question #7
Solution Student's Question:
This question is about Pre-test Monitoring and Controlling question #7 and is about float, free float and total float:

In the question and the rational for the right answer it says that:
1. Free Float is: =Earliest Start of Following Activity-Earliest Start of Present Activity-Duration of Present Activity. And according the definition the free float this is amount of time an activity can be delayed without delaying the early start of its successor.
2. From other notes the definition for Total float is that it affects the end date of the project.

What do you calculate with the following formula LS-ES or LF-EF?.
Is the called simply “float” for a specific activity?
How do you calculate total float? Do you just sum the float of different activities?
Is it possible to use the same formula (LS-ES or LF-EF) to calculate the free float, because when I do it looks like I end up with the same result?

Expert's Answer:
We need to be careful to understand the differences between free float and total float. Total float is the amount of time that an activity can be delayed without having to reschedule the project completion date. Free float is the amount of time an activity can be delayed without having to reschedule the ES of any other activity in the project.

Total Float is shared with all of the other activities in the same path. Therefore using the float in one activity will reduce the float available for the other activities in the path. It is the difference between an activity LF and LS or EF and ES. You do not sum all the float for all activities in the path to get the total float.

And then we have Free Float, which is the amount of float that the activity can consume without affecting the earliest start of the successor activity. Only activities that precede a junction can have free float.

Draw out this example below with just 3 activities and you will see the difference between total float and free float.

Task A and B must complete before Task C can start.

Task A ES = 3; DU 1 EF = 4; LS = 22; LF = 23

Task B ES = 10; DU 2 EF = 12; LS = 21; LF = 23

Task C ES = 12; DU 3 EF = 15; LS = 23; LF = 26


Task A total float is 19

Task B total float is 11

Task C total float is 11


Task A is a junction to Task C with a free float of 8 and a total float of
19.

Free float is calculated by: ES of Task C (12) - ES of Task A (3) - DU Task
A (1) results in free float of 8 even though the total float for Task A is
19. Naturally free float could never be larger than the total float.

 

Article Details
Article ID: 128
Created On: 07 May 2008 09:11 AM

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