While practically speaking, it may be a matter of choice, but from the exam point of view, there's only one correct approach. So, what is that correct approach? Should we start the forward pass calculation at zero or one? Today I'm going to answer that question for you, support it with evidence and put an end to this debate. So, are we ready to dive in?
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Figure 1 (Approach 1 - start at zero)
Figure 2 (Approach 2 - start at One)
In figure 1, we started with zero and performed the forward and backward pass calculations. In figure 2, we started with one.
- In both the cases, the Critical Path duration and float calculations yield the same result
- The formulas to calculate the ES, EF, LS, LF, and FF are different.
- For calculating the duration of critical path and float, it doesn't matter which approach you take as long as you use the right formula.
- The values of ES, EF, LS and LF are different in the two approaches.
- The formulas and calculations are relatively simpler in approach 1 (but your preference may be different).
The Correct Approach
So, the (million-dollar) question is, which approach is right from the PMP exam point of view? Which approach should I use in the PMP exam?
Update 04 March 2017: This article was first posted about 5 years ago (Mar 2012). Based on the feedback in the comments and other changes that have happened since then, I'm striking out the section below. There's no official word from PMI as to what the correct approach is. If I have to pick one of the two approaches at this point, I would go with Approach 2 as that's what is used in the PMBOK® Guide, 5th Edition.
Yes, as far as the PMP exam is concerned, the correct approach is Approach 1, and that's what you should be using on the exam. What you need proof? You don't trust me? Okay fine, listen straight from the horse's mouth:
PMI Community Post - Critical Path: Start at Zero or One
I quote the following passage from that article:
If you are sitting for the Project Management Professional (PMP)® certification exam, remember Task A always starts at zero. It has no predecessor and, thus, can begin right away. You will get incorrect answers as you do the backwards path and float calculations otherwise.
Now you might ask, why some authors and course providers use Approach 2. Well, that's for them to explain.
Hope we have settled this Start with Zero or One debate for good.
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