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TOPIC: Free Float Calculation

Free Float Calculation 10 years 8 months ago #3055

  • Edward Wada
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I had purchased your Formula Study Guide. For Free Float Calculation is had
Free Float = Earliest ES of Following Activities - ES of Present Activity - Duration of Present Activity

I had several questions:
1) Formula
I noticed sometimes the formula would be listed as
Free Float = ES of next activity - EF

It seems this is slighly different? Why would this be different?

2) Formula based on PERT method
Would the Free Float formula differ base don the PERT method? For example PERT with the First day being "Day 0" or " Day 1".
Since you use Day 1 as the first day, you would use your formula, however the same answer would apply if using the alternative formula if the First Day is "Day 0".

If not, please clarify.

3) Free Float vs Total Float
Would it be true to say, the formula for Total Float calculation would not change if you use First Day "Day 0" or "Day 1"?

Thank you for the clarification. :silly:

Free Float Calculation 10 years 8 months ago #3056

  • Khurram Hussain
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Dear Edward:

Good day!

For your first question, you can use either of the formulas to calculate the free float. Both formulas are correct. I am explaining the reason below, however I recommend just pick up one of them and stick with it till you pass your exam. Here is the proof that both formulas are the same:

We know that, EF = ES + Duration, right? Now let’s start with your first formula and prove the second formula from it:

= ES of next activity - ES of present activity - Duration of present activity
= ES of next activity – (ES of present activity + Duration of present activity)
= ES of the next activity – EF of present activity

For the second and third questions: These formulas are correct if your starting point is zero. If your starting point is 1, the first formula will remain the same. However, the second formula will change to:

Free float = ES of next activity – EF of present activity – 1

I highly recommend using the “day 0” approach as it resolves all of these confusions.

Let me know if this resolves your confusion.

Warm regards,

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