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TOPIC: CPM Calculations - Start with day 0 or day 1?

CPM Calculations - Start with day 0 or day 1? 3 years 6 months ago #3349

  • Abhinav Gupta
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For the exam does it matter whether you start with day 0 or day 1 for your CPM calculations? The book I am referring uses day 0, while the prepcast uses day 1. While float and critical path will remain the same using both methods, the ES & EF dates might differ. I prefer using 0 as the first day as it is not only faster but also reduces the chance of mistakes!

Your thoughts?

CPM Calculations - Start with day 0 or day 1? 3 years 6 months ago #3353

  • Cornelius Fichtner
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This question has been asked here in the forum about 5 times so far. 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.
Until Next Time,
Cornelius Fichtner, PMP, CSM
President, OSP International LLC
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CPM Calculations - Start with day 0 or day 1? 3 years 6 months ago #3361

  • Abhinav Gupta
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Hi Cornelius, I read this post in your voice :P

Unfortunately, this does not answer my question regarding the ES & LS dates. Using either technique, it is clear that the float values will be the same. However, if you use the day 0 method, the ES & LS dates are different from the ES & LS dates obtained using the day 1 method. I have found that you have to add 1 to the ES & LS dates in the day 0 method, you get the same dates as those in the day 1 method.

While PMI does not prefer any particular technique, what do you do if a question on the exam asks "What is the ES of activity D"? Clearly there would be two possible answers for this question.

CPM Calculations - Start with day 0 or day 1? 3 years 6 months ago #3367

  • Cornelius Fichtner
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Abhinav,

If PMI had such a question on the exam (which I strongly doubt) then they would make sure that there could be "no" discussion about the answer. So let's say that the ES your activity D is on day 4 or day 5 (depending on which method you used to do the forward/backward pass). Then PMI would give you the following options to select from:

A.) Day 1
B.) Day 4
C.) Day 8
D.) Day 12

As you can see the answers are far enough apart so that B is the only "logical" answer.

And this is exactly my point: PMI will probably not have such a question on the exam because depending on the approach you choose you get a different answer. They know about this. They will avoid this, because they don't want students to come back again and again about this question contesting it.
Until Next Time,
Cornelius Fichtner, PMP, CSM
President, OSP International LLC
Last Edit: 3 years 6 months ago by Cornelius Fichtner.

CPM Calculations - Start with day 0 or day 1? 3 years 6 months ago #3369

  • Abhinav Gupta
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Aah that makes sense. Thanks a lot for clarifying Cornelius.

Until next time! :)
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