TOPIC: Calculating Schedule Duration

I have a question about calculating schedule duration. Prepcast gives the following formula: LF-LS-1 or EF-ES-1 but when I used that formula taking tests from Kim Helman's books I got the question wrong. She uses the formula EF-ES or LF-LS. Which is correct?

Thanks

Dear Christy:

Good day!

That is a great question!

Both methods are correct. I strongly recommend learning one and sticking on to that method. However let me explain why these two methods differ.

The formula differs in these two methods as the first method assumes the start of the first activity as ‘0’, while the other assumes the start of the first activity as ‘1’ (1st day).

Let’s assume we have three sequential activities A, B, and C.
A: Duration=10 days;
B: Duration = 5 days;
C: Duration = 15 days;

Method 1:

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Let’s calculate the Early Start and Finish dates for each of these sequential activities by the first method (i.e., that assumes the start of the first activity as ‘0’):

A: Early Start=0; Early Finish=10; (In this method: EF = ES + task duration)
B: Early Start=10; Early Finish=15 (In this method: ES of this task is the EF of the previous task)
C: Early Start=15; Early Finish=30;

To calculate the durations of the tasks you will use the formula EF-ES. Try it, you will get the same durations.

Method 2:

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Let’s calculate the Early Start and Finish dates for each of these sequential activities by the second method (i.e., that assumes the start of the first activity as ‘1’):

A: Early Start=1st day; Early Finish=10th day; (In this method: EF = ES + task duration - 1)
B: Early Start=11th day; Early Finish=15th day; (In this method: ES = EF of the previous task + 1)
C: Early Start=16th day; Early Finish=30th day;

In this method, to calculate the durations of the tasks you will use the formula EF-ES+1.

If you stick to one method, you will get the correct and same results. However, if you mix them together, you will get incorrect results. Please take a question from both books (showing different methods) and solve both questions with both methods. You should get the same results.

I personally recommend using the first method, as it frees you from +1 and -1 based formulas.

Do let me know if this helps you clear your confusion.

Regards,

Khurram Hussain, PMP, CSSBB

Thank you Khurram Sir for explaning it so nicely (with examples). I think after reading your post, anybody can understand the difference between the two methods.

Actually I was planning to reply the question (after reading just the question) but honestly my reply will say that difference is if you take the start as "0" or "1". It will NOT take the examples as you did. Even I used the +1 or -1 formula but after reading your post, it seems like the other one is much easier.

One more question though, as you are the moderator. Although I purchased PMP Simulator end of last month, but recently started the practice exams. Can I ask questions from the Simulator in this forum or there is a separate forum for the Simulator (I don't believe so but thought its better to ask). Also can we copy paste question/part of question form the Simulator here or not (I mean copyright issue). I remember you replied one of my earlier post that its better to ask the concept in lieu of the exact question itself but sometimes I am feel little difficult as how to ask (but sure I can try). Is the feedback feature of the simulator does the same thing?

Thanks and Please reponse as per your convenience.

Regards, R.

Dear Rachu:

Good day and thanks for the appreciation.

As you have said, you can ask the questions regarding the concepts here and should not copy the actual questions. Yes, you can use the feedback feature if you notice anything confusing with the question. Once you leave a feedback on a question, it will get reviewed and one of the experts from the company will reach out to you soon.

Thanks,

Khurram Hussain, PMP, CSSBB

Thank you and I shall use the "feedback" feature then.
Regards, R.