When I was preparing for my PMP exam, I was very confused about the difference between the two CPM concepts - Total Float and Free Float. I understood Float, but not the two types. So, I did some research and finally made-up an example to understand the concepts myself. Today I'm sharing the example that I prepared almost 2 years ago. I hope it will help PMP aspirants on their exam.
Let's look at the PMBOK Guide definitions first.
Total Float: The total amount of time that a schedule activity may be delayed from its early start date without delaying the project finish date, or violating a schedule constraint.
Free Float: The amount of time that a schedule activity can be delayed without delaying the early start date of any immediately following schedule activities.
Now we'll look at the example. Refer to the Network Diagram in the figure first. I'm assuming that you know how to work with a Network Diagram and how to calculate Early Start, Early Finish, Late Start and Late Finish for activities on a Network Diagram.
Problem: What is the Total Float and the Free Float for Activity F and J?
Solution: Formulas:
Total Float = LF - EF (or LS - ES)
Free Float = ES of next activity - EF
The calculations are really simple.
For Activity F:
Total Float = LF of F – EF of F => 8 – 5 = 3
Free Float = ES of G – EF of F => 5 – 5 = 0
For Activity J:
Total Float = LF of J – EF of J => 8 – 3 = 5
Free Float = ES of G – EF of J => 5 – 3 = 2
Note: Free float can only occur when two or more activities share a common successor, or in other words, when activities converge on a Network Diagram. In our example, only activities F and J can have Free Float.
Hope you'll do well if you get such a question on your exam. If you have any doubts, I'm here to help.
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7 comments:
Now that you have done a fine job clarifying the differences between Total and Free Float, how would or could or should a professional project manager USE free and total float?
Who owns total float?
Who owns free float?
My real complaint with the PMP exam (and PMI's approach in general) lies with the fact that while people may know the terminology and in some cases, may actually know how to do various calculations, they haven't a clue as to how to effectively use this information to manage projects.
BR,
Dr. PDG, in Boston
http://www.getpmcertified.com
Hi Dr. PDG,
Once again, you have challenged us 'mortals'.
I'll give my short answer and let you elaborate.
Total Float belongs to the project, whereas Free Float belongs to individual activities. If the project is being done under a contract, it's debatable whether Float belongs to the buyer or seller.
In either case, 'Float' is a project resource. It should be used judiciously, mainly to cover the risks or other unforeseen issues on the project.
Now I pass the ball to you. Please enlighten us more on this subject.
2. Regarding your point on the PMP certification, I think we are on the same page. Nobody is claiming that PMI-certified PMs are "Super PMs". I see it more as an academic qualification (similar to a school or college degree). Again, I have had this debate so many times in various forums and really tired of it. There's no end to it ...
Thanks once again. I appreciate all your inputs.
Not too bad an answer, BW!!! I am impressed.
From a claims (legal precedent) perspective, unless otherwise stated in the contract, float is generally considered to be owned 50% by the owner and 50% by the prime contractor. However, the 50% owned by the prime contractor is also split between his/her subs and vendors.
And although you are absolutely correct that float is to cover or mititage risk, a better or more complete answer would be that it shows you how much flexibility you have in terms of both time (spread between early and late date S curve horizontally) and in terms of resources (the spread vertically between the early data and late date S curves)
Very few people, unless they have been involved in litigation, truly understand how to effectively use float information to better manage their projects.
BR,
Dr. PDG, in Boston
http://www.getpmcertified.com
PS: For those really interested in this topic, be sure to download the many free Recommended Practices (RP's) AACE offers. http://www.aacei.org/technical/rp.shtml . Pay special attention to 29R-03 to learn about many of the games contractors play with schedules.
And if you are interested in project controls/PMO as a career path, I urge you to move beyond PMI's rather sophomoric credentials and challenge yourself to some very difficult, albeit less well marketed credentials that AACE offers. http://www.aacei.org/certification/
BR,
Dr. PDG, in Boston
http://www.getpmcertified.com
Nice explanation ...
confused OR student
@ Anonymous:
That's an excellent observation. I have answered this question previously in a forum too, but can't dig out the link right now. I'll answer it here again.
You are right. Different books follow different conventions. Whether you start the first activity at 0 or 1 is a matter of choice. Both methods will get you the same answers. It doesn't make any difference when it comes to calculating the duration of critical path, and float values.
I prefer to follow the convention that which you find in Rita's book, to calculate the ES, EF, LS, LF, FF, TF.
The advantage with this convention is that you don't need to add/subtract hours. The first day starts at 0. If the first activity's duration is 7 days, then its ES = 0 and EF = ES + Duration = 0 + 7 = 7. The successor activity will have the ES = EF of the predecessor activity (provided there's no lead or lag).
In summary, pick the convention that you are more comfortable with, practice it, and stick to it.
Hope it answers your question. Thanks for bringing it up.
Harry, i was bit puzzled to use 0 or 1 to calculate the ES of the first activity.. But after seeing your explanation, now i am clear on using 0 or 1. Thank you
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