Critical Path Forward Pass Calculation - Start at Day Zero or One?

Excellent article Harwinder!! This was precisely the debate that was ensuing hotly in the PMP bootcamp batch I am participating in my local city. The trainer was using the HeadFirstPMP book as one of the training materials and it uses the Approach 2 (forward pass - start at one), whereas she was insisting that according to her own experiences with the PMP exam and feedback from previous PMP bootcamp candidates, the correct answer is Approach 1 (forward pass - start at zero). Thus, the confusion arose and a lively debate ensued, with everyone voicing their favorite book's recommended strategy. I wondered how PMI can allow the popular PMP book authors to be so contradictory on such a fundamental topic! Can't the PMBOK be more specific on the Critical Path Methodology. It seems to rec Thank you so much for shedding the perfect light on this very critical subject and confirming it with that PMI post link, now I can't wait to share this tidbit with my batchmates tomorrow. :-)

Wrong answer, folks...... To make a long story short, the clock starting with zero is the mathematical method used by ADM (Activity Diagramming Method)while the clock starting from 1 is used by PDM (Precedence Diagramming Method). Given that only one major software package uses ADM (Microplanner) and the last time I used it commercially was back in 1983, pretty much tells the story. If you look at MS Project, Primavera and all the major software tools in use today they all use PDM. Some time ago, PMI rationally and logically dropped ADM in favor of PDM meaning when you start calculating float, start with 1, not 0. BR, Dr. PDG, Jakarta, Indonesia http://www.build-project-management-competency.com

WOW. New Look of your blog is awesome. Now looking very cool.

Hi Harwinder, In case of "Free Float" calculating, it will give different result if you start with 1 than 0. Anyway, my understanding is that starting with 1 is more realistic as first date cannot be 0 (keeping in mind calender date). Otherwise it's fine using any of them. Regards, Mrinmoy

Hi Harwinder, In the "old days" PMI used both ADM (clock starts running at time 0) and PDM (clock starts running on the morning of day 1) but starting with the PMBOK Guide 3rd Edition, PMI dropped ADM and from that point forward, all the calculations started using PDM, meaning the clock starts running on the morning of day 1. Put it this way- this is the method I teach in my classes and we have close to a 20 year running first attempt pass rate of 86% for English as Second Language (ESL) practitioners and 99%+ for native/near native speakers of English. BR, Dr. PDG, Jakarta

I'm still confused. I've seen BOTH ADM & PDM in questions. Granted some questions might have been from out of date exams, but I'm grabbing any info I can get. I would HOPE that the question on the exam would specify which to use.

Dear Harwinder How you co-relates activity versus tasks in project management.

I was disappointed that I had to read to the end of the article to get the answer. I would have preferred the correct answer be stated at the top to save my time; as well as that of whoever else found themselves drawn here. The icing on that cake turns out to be that the article's conclusion is wrong anyway. Thankfully, I read into the comments to find that out. From Mr. Harwinder's response to his commentors, it turns out he is loath to rescind his erroneous conclusion or ammend his article. It seems Mr. Harwinder is more interested in reveling in the controversy than settling it; and it appears the real purpose of this article is to perpetuate it.

Great article. Thanks for explaining.

Great article. FYI, PMI have now commented on this topic via an errata to the 5th edition of the PMBOK.> Pg 177 Figure 6-18--Added the following clarifying the note to the figure. This example uses the accepted convention of the project starting on day 1 for calculating calendar start and finish dates. There are other accepted conventions that may be used.

Nice article! Good approach to explain the difference between the ADM(clock starts at 0) and PDM (clock starts at 1). But - to avoid any further arguments and confusion to the topic, it would be great if the section that "declares the clear winner" is modified or deleted altogether. Personally, while understanding the concept myself and explaining others-the PDM method,that starts with Day 1 was preferred by me. My rationale - 1. This method (PDM) one of T&Ts uses for calculating CPM in a network diagram - appearing 2. Furthermore, if you look at it from a practical approach, our calendars start from Day 1. Don't really see a "Day 0" in our calendars-do we? 3. PMBOK version 5,page 177 (figure 6-18) CPM example also takes start date as day 1 and mentions it as an accepted convention. In the same figure,the foot-note also says that there are other conventions to express the start date. Since PMI itself is avoiding to prefer any given method(between PDM and ADM), it would be better if the publisher can modify the blog to avoid confusions on this topic.

BTW - the PMI.org community article link has now been removed

Greetings, The debate continues. "A User's Manual to the PMBOK Guide" (updated for the 5th Edition PMBOK) which is published as a companion for the PMBOK uses the starts at zero.

Hey Harwinder, thanks for the article. This was really insightful. I just have one follow up question: What if you were given just an activity node with ES and EF or LS and LF and were asked to calculate the Duration. Which of the two methods would you use? I ask this because, in that case the calculation of duration (EF-ES) would be different for both methods. I would really like your feedback on this. Thanks again for the blog!

Thanks for yours clarification on both approaches, what if the duration in weeks instead of the days. I don't think we can use 2nd approach while duration is describe in weeks. Please explain.