There are indeed 2 approaches:
- First approach: You calculate the network diagram starting on day 0
- Second approach: You calculate the network diagram starting on day 1
In the PM 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 and 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 2nd approach).
Of course, this often leads to confusion for students taking the Project Management Professional (PMP)® exam 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 the Project Management Institute (PMI)® does not "support" a specific method of calculating a network diagram. (Remember that next to the 2 options shown above you could also calculate a network path starting on a specific calendar date in hours instead of days, making calculations even more complex).
Neelesh Pandey, PMP (who is 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, LF for a simple network diagram using both the methods.
As you see: Both of these calculations will lead you to the correct answer. The big difference for the exam, however, is that the first approach (starting on day 0) involves less calculation because you don't have to "+1" all the time. So in order to reduce your "risk" of doing a calculation wrong on the exam you may want to do it starting on day 0. However, in "real life" starting on day 1 is more appropriate.
The conclusion is that PMI is aware of these varying methods and that you should not see a question on the exam where only the application of any particular method leads to the right answer.