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TOPIC: Critical Path, Forward Pass & Backward Pass

Critical Path, Forward Pass & Backward Pass 5 years 4 months ago #2231

  • Adam Leslie
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I just finished the PM Precast episode "Network Diagram" E06.07. Good episode, and well explained.

However, I have one question: My understanding it that one should calculate the f-pass and b-pass on the critical path FIRST. This is especially important since the LF of the final critical path node is the late finish for any other final nodes (if any). Is this incorrect?

Thanks!
Regards,

Adam

Re:Critical Path, Forward Pass & Backward Pass 5 years 4 months ago #2238

  • Cornelius Fichtner
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Adam,

There is no way to calculate the f-pass / b-pass on the critical path first, because you FIRST have to calculate the f-pass / b-pass to determine what the critical path actually is.

If you look at an empty network diagram there is no way to determine the critical path. That's what f-pass/b-pass are for.

Are you confusing this with "Always crash/fast-track the tasks on the critical path first"?
Until Next Time,
Cornelius Fichtner, PMP, CSM
President, OSP International LLC

Re:Critical Path, Forward Pass & Backward Pass 5 years 4 months ago #2239

  • Tarek Al-Fahham
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After I've been throuhg this topic several times, it apears to me that f-pass/b-pass are necessary, not only for calculating the Critical Path, but only necessary to calculate the Free Float and Total Float.

This is because you can calculate the Critical Path using another method which does not require f-pass/b-pass, which is simply to add-up the duration for all the possible paths and the longest one is the Critical Path.

Appreciate to tell me if my understanding is correct.

Tarek.

Re:Critical Path, Forward Pass & Backward Pass 5 years 4 months ago #2240

  • Adam Leslie
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Not at all!

First, you can determine the number of paths simply by looking at the diagram. You can also add up the arrows. Count the number of arrows leaving the "start" box, and every time an activity node explodes into two, add another path (or if it explodes into three, then count two - and so on). Now you know how many total paths.

Next, identify the activities in each path (i.e. Path 1 = A,B,D; Path 2 = A,B,C). Now just add the durations of each activity node in each path ... and violia! You have just found the critical path! It's the path with the highest number.

And now I can get float too! Simply subtract the next highest path from the critical path. Say you get the number "2". That is now the float for every activity on that path (unless a node is already filled in from a previous path).

That's it! I have critical path AND float and never calculated using FP/BP. I also use this as a check when I do FP/BP, because the floats should obviously equal each other.

Is that clear as mud? :)
Regards,

Adam

Re:Critical Path, Forward Pass & Backward Pass 5 years 4 months ago #2243

  • Tarek Al-Fahham
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Yes you are right. Actually, I mistyped my origianl statement. I wanted to say:

"it appears to me that f-pass/b-pass are necessary, only and only if you want to calculate ES, EF, LS, LF, Free Float and Total Float for each activity".

In that case, using start as Zero or One (using or not using +1 and -1) in the formulas will give different result to ES, EF, LS, LF, but same result to Float.

I hope now my understanding is correct.

Tarek.

Re:Critical Path, Forward Pass & Backward Pass 5 years 4 months ago #2244

  • Adam Leslie
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Just remember, if you use the method above you DO NOT have to do a FP/BP to calculate float! Plus, you can use the method to check your work when you do perform a FP/BP ...
Regards,

Adam
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