One of the most frequently asked and hotly debated topics about Critical Path calculation by PMP aspirants is whether to start the forward pass calculation at Day Zero (0) or Day One (1). This has been a topic of debate on numerous forums and blogs, including this blog. But I've not seen a conclusive answer anywhere. The PMP exam prep guides and courses add to the confusion by following different approaches. Some authors prefer to start at zero, some start at one. Each approach yields different results.
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?
First, let's consider a simple example and see how the results differ when we follow one approach over the other. Refer to figure 1 and 2.
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?
And the winner is ............................ Approach 1 - Start at Zero (0)
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.
If you have questions or comments, please feel free to post them below. I'm always eager to read your comments. While we are here, I also want to invite you to join my new forum PM Hangout. All I'll say is if you love this blog, you'll love the forum too. So, what are you waiting for? Join now: