TOPIC: Free Float

Dear all ,

I need your help to understand Free Float Calculation : In some sources formula

FF = ES (successor) - EF(predecessor)

and in others sources

FF = ES (successor) - EF(predecessor) -1

Thank you

No one can respond ?)))

Either formula can be correct. It depends on how you set up the network diagram. If you refer to page 211 of the PMBOK® Guide, you will see that the network diagram uses the convention of the project starting on day 1 for calculating the start and finish dates. You will also see a note which refers to other conventions that may be used. Another way of setting up the network diagram is to use day 0 instead of day 1. With the day 0 method you will notice that the ES and LS do not advance a day from the previous node on the critical path. With the day 1 convention, the ES and LS advance to the next day. Since the PMBOK® Guide uses the day 1 convention, I would suggest using this same method during the exam. When using this method you will need to add or subtract one in the formulas.

This question has been asked here in the forum many times. Here is once again my standard answer, copied from our PMP Exam Formula Study Guide, which you can find at www.pmformulas.com :

There are two approaches for calculating ES, EF, LS and LF:

• First approach: You calculate the network diagram starting on day 0
• Second approach: You calculate the network diagram starting on day 1

In the PMP Exam Formula Study Guide we use the second approach, because when your sponsor tells you that your project starts on the first day of September, then that is September 1, not September 0. This is also the way that all modern scheduling tools seem to work. You schedule your project based on a calendar start date and not "on day 0".

That is why there is a slight difference between the calculations: you have to add/subtract 1 from the results in the second approach.
Of course, this often leads to confusion for PMP exam students and they ask which formula should we use on the exam?

We have discussed this with a number of PMP trainer colleagues and they agree that PMI does not "support" a specific method of calculating a network diagram. (Remember that next to the two options shown above you could also calculate a network path starting on a specific calendar date in hours instead of days, making the calculations even more complex).

Neelesh Pandey, PMP (a PMP trainer) has told us the following about his teaching experience with these formulas:

I use a PowerPoint presentation with animations to prove that no matter what method you follow, the result is same. I choose a part of a network diagram with four sequential activities, which sum up to a duration of 10. This path has a float of two based which we calculate LS and LF. My participants once assured that it doesn't make a difference tend to use the "zero" method. Somehow they find it simple as no subtraction is needed. PPT animation helps me a lot and also I ask my participants to calculate ES, LS, EF, and LF for a simple network diagram using both the methods.

As you see, both of these calculations will lead to the correct answer. However, in the exam the big difference is that the first approach (starting on day 0) involves fewer calculations because you don't have to "+1 or -1" each time. So, in order to reduce your "risk" of doing a calculation wrong and saving time during the exam, you might want to initiate the network diagram with day 0. However, in "real life" starting with day 1 is more appropriate.

Since PMI is aware of these varying methods, you should not see a question on the exam where only the application of one or the other leads to the correct answer.