Deep Fried Brain Project - PM Certification Exam Prep Blog: Total Float vs. Free Float in CPM

Total Float vs. Free Float in CPM

Posted by Harwinder on Monday, August 03, 2009 - 21 comments | Readability Mode

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.

Related Posts:

21 comments:

Dr. PDGAug 3, 2009 04:19 PM
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
ReplyDelete
Brain WasherAug 4, 2009 05:26 AM
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.
ReplyDelete
Dr. PDGAug 4, 2009 06:26 AM
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
ReplyDelete
Dr. PDGAug 4, 2009 06:33 AM
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
ReplyDelete
MSDec 7, 2009 01:55 PM
Nice explanation ...

confused OR student
ReplyDelete
AnonymousJan 16, 2010 01:11 PM
I have a related question, which I m slightly confused about. Each of the books Rita, Head first and others have 2 different ways of drawing the network diagram..e.g Activity A starts on Day 1 and has a duration of 5 then the ES of the activity A is 1 and the EF is 6....and Activity B which proceeds activity A has a duration of 8, then Activity B, then the ES for B is 6 or 7 considering there is no lag between A and B.

Same way while calculating LF and LS some authors subtract a day by default others don't...which is confusing...If you can kindly help with this, with an example that will be very helpful.

In the end for calculating float it doesn't make any difference as adding 1 and then subtracting it correspondingly adjusts for the same.

What is the standard practice for this, can you please clarify?
ReplyDelete
HarwinderJan 16, 2010 07:58 PM
@ 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.
ReplyDelete
ChelluriJan 19, 2010 06:45 AM
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
ReplyDelete
SOUMITMar 27, 2010 08:33 PM
If an activity x, ES = 1 and duration =5, EF = 1+5=6.
ES for activity (x+1)=6.

However, I have seen calculations, where EF(x) will be 5 (1 through 5 = 5days). then ES(x+1)=6 (not 5).

Both these methods will not make any difference to your CPM calculations as long as you keep it constant throughout.
But, in ur PMI certification exam, this can make a difference because you have to choose what PMI thinks is correct from the given options.

Use the latter I think.
ReplyDelete
williams rodriguez constantinoJul 13, 2010 11:20 AM
Definitivamente son terminos que son necesarios comprender a fondo para darles la aplicacion correcta en nuestros proyectos, agradezco mucho los comentarios y los felicito... :D
ReplyDelete
HarwinderJul 13, 2010 06:01 PM
Hello Williams,

I cannot read Spanish. So, I translated it to English for others' and my own benefit. Here's what it translated to:

"They are definitely terms that are necessary to fully understand them correctly applied in our projects, I very much appreciate the comments and I congratulate ... : D"

Thanks for your comments :)
ReplyDelete
RookieSep 14, 2010 06:41 AM
Continuing from Soumit's comments on 28 March, I'm sharing this link and a few questions.
http://www.microknowledge.com/trainingFileDL/pmTrn/CPM%20Calculation%20Template.doc

This's the clearest description of the 2 conventions by far. It says:

The Zero Method – all starts and finishes are at the end of a time period. In the Zero Method, the start of an activity has the same number as the finish of the previous activity on the critical path.
The One Method - all starts are at the beginning of a time period and all finishes are at the end. In the One Method, the finish of a previous activity on the critical path, has 1 added to it to get the start of the next activity.

It seems that
- Rita Mulcahy uses the "zero method"
- Andy Crowe uses the "one method"

So 2 questions remain
1) Which one does PMI use? Any recent exam takers care to share your experience?
2) Are "zero method" and "one method" the official terms to describe these calculation conventions?
ReplyDelete
AnonymousDec 15, 2010 03:29 PM
the formula for FF = ES of next activity - EF is wrong
it should be FF = ESn - ES - duration
ReplyDelete
HarwinderDec 15, 2010 04:42 PM
Hello Anon.,

Thanks for your feedback.

Please elaborate on the source of this information.

BR.
ReplyDelete
AnonymousJan 7, 2011 08:33 AM
Another excellent source of information on PMP is the PM Road Trip and in particular the calculation for free float as put forward by anon is correct:
FF = (ES of successor - ES of current - duration of current)

This can be found here:

http://www.pmroadtrip.com/pdm_01.html

I prefer the "one menthod" as it is reflects the real world much better. I would never start the first activity on day zero as that doesn't make sense. You start on day 1 and if it takes 5 full working days to complete (duration) then you would be working on it on days 1, 2, 3, 4, and 5. Then the earliest you can start the next activity is day 6, not day 5!.
ReplyDelete
ShraddhaJan 24, 2011 09:06 AM
This comment has been removed by a blog administrator.
ReplyDelete
HarwinderFeb 7, 2011 07:26 AM
Hello Shraddha,

I had to remove your post because it violates PMI's code of conduct. Try to rephrase your question and post it again.

Thanks.
ReplyDelete
AnonymousFeb 27, 2011 03:38 PM
I think both of th efollowing are same
FF=ES of next activity -EF of current - (1)
FF = (ES of Successor -ES of current -Duration of current) - (2)

here is the explaination
From (2) above
FF =(ES of Successor -ES of current -Duration of current)

which means
FF =(ES of Successor -(ES of current +Duration of current))

and -(ES of current +Duration of current) = EF of current

so it becomes

FF =(ES of Successor -EF of current)
whcih is same as (1)

so formula is harry's post is same as what annon has posted.
ReplyDelete
AnonymousApr 8, 2011 09:54 AM
Hi,
I have another question regarding identifying float and utilizing it as a buffer/slack.
In case of leads and lags to N/W diagram, how should float be identified? For example, it might be required to wait for 2 days before Activity B starts after Activity A, say due to planned maintainance. If we just go by float calculation, this may be considered as a buffer that can be utilized.
So how to really identify fload in case of leads and lags?
ReplyDelete
MuhmadJul 4, 2011 04:43 AM
Thanks harwind for your wonderful example
ReplyDelete
RamSep 18, 2011 07:30 AM
Harwinder,

I would like to know about Project float ( Rita's book).As per my understanding there is no float for critical path activities.If this is correct.Does Project float existis?.

And How do we diffrenciate float with leads and lags!

Thanks
Ram
ReplyDelete