Carrying out critical path calculations within the PMP exam (or any other examinations), can seem like a daunting exercise, but it is not. In fact, as long as you have a calculation SYSTEM, you really can’t go wrong!
CPM is just being about to add up and subtract!
You see, it is just a matter of breaking the calculation into two steps:
Out of those two steps, it’s just a matter of pulling out the various answers that the question is asking for…
There are some rules or laws for using the critical path method, but other than that, it is just a matter of calculating carefully and logically.
Rather than giving you a written explanation, grab yourself a pen and paper, and jot down some notes as you watch this video:
Feeling confident that you understood those simple rules?
Here is an example I was given by one of my students, who needed to answer the following question:
Let me answer this critical path analysis example in a series of steps so you can clearly see the calculations as they unfold:
The first step in the critical path analysis example is to carry out the forward pass as explained in the video. Just follow the forward pass rules and add the durations of each activity:
Here you can see that I’ve calculated the earliest finish ‘date’ for the project in just the forward pass – 17 time units.
Because the critical path consists of the numerical addition of all the activity duration’s of the critical activities, and because the critical path is the LONGEST path through the network, then the end date as above is also the sum total of the critical path activities.
So far, it’s not so hard is it?
Again, keep a cool head, all I’ve done here is to follow the backward pass rules as explained in the video, and SUBTRACT the duration’s where required.
So here’s the start of the fun part….
Calculating the critical path
You can establish the critical path – just by observation!
Just check out EITHER the front vertical pair of numbers OR the back vertical pair of numbers.
Use the difference between them to calculate the Total Float.
Any activity where EITHER the front pair OR the back pair have the same numbers – then these are critical tasks!
Any of the activities that have a difference between EITHER the front pair or the back pair is a non-critical task AND the difference
…..Is The Total Float!
That makes the whole job very easy as shown in the following diagram:
The final part in the question, really gets to demonstrate your understanding of not only what the critical path method is calculating, but what it actually means – in other words, this last piece checks that you can analyse the results…
So, what the question asks, is if activity F was removed, what are the consequences or impact?
If ever you see such a question, don’t be tempted to make a quick visual assumption.
Instead, re-do the calculations starting with the forward pass and finishing with the backward pass calculations as above.
Check out what has happened:
1. Two of the paths from Start to End are now critical. This means that their combined path duration’s are the same!
2. This means that their are now only THREE activities that are non-critical
3. This ALSO means that apart from those three activities, if ANY of the critical tasks are either delayed or their duration’s increased, then that same delay will transfer to the project end date.
4. Put another way, only activities A, B, and C, have any float (since critical tasks have zero float), and
5. Like any float applied to tasks in series, it will be shared between them all, so activities A, B, and C, share one day total float!
I sure hope that this article has given you great critical path method skills and confidence in answering such questions!