Tuesday, September 26, 2017

PMP Protein: Critical Path Method – Basics

By Manas Das, PMP




Critical Path Method (CPM) is known as one of the key schedule network analysis techniques in “Develop Schedule” process of Time Management knowledge area, under planning process group. 

This analysis is applied on the schedule network diagram, which is created in “Sequence Activities” process – also in Time Management knowledge area and planning process group.


The simplified diagram with the only the key input and outputs, along with the CPM technique is shown below. 




What is Critical Path?
Let’s see the definition of critical path. 
“Critical path is the shortest possible duration within which the project will be completed or critical path is the longest path in the network.”


Above two lines defining critical path may look contradictory to you, but they are not! It means if the critical path is delayed, then the project’s end date will be pushed. Hence it is the shortest possible duration within which the project will be completed. The other aspect – longest possible path, informs us that if you delay on this path, of course the project duration will be elongated. 

It’s worth to understand that activities on a critical path are critical from schedule point of view only and not from functionality or complexity point of view. Project management practitioners, who are new to this concept, confuse on this. All activities on the critical path are known as critical activities. 

Again, the activities are critical, because if you delay any activity on critical path, the project end date will be pushed further. 

More the number of critical paths, more the risks to the project. Let’s check this scenario. Imagine you have a schedule network diagram with multiple critical paths. As it has many critical paths, you can say that there are multiple ways in which the schedule can be delayed. Hence, more risks. 

Let’s take an example to understand more on it. Below, a simple network diagram is shown with various activities. The duration of the activities is in days, e.g., Activity A is of 5 days duration.




There are 3 different ways to complete the tasks which will take different time to complete as below.
Path -1: Start – A -  B – D – F – End = 16 days
Path-2: Start – A – B – E – F – End = 15 days
Path-3: Start – A – C – E – F - End =11 days

But Critical path is Start – A – B – D – F - End being the longest path which takes 16 days to be completed.

Second Definition of Critical Path 
There is another definition of critical path as well, which you can see while using project-portfolio management tools such as Microsoft Project or Oracle Primavera. It goes as follows. 

“Critical path is the network path in which the total float of activities can be less than or equal to zero.”

Total Float and Free Float
To understand the second definition of critical path, first it requires to understand what total float and free float.

There are two types of floats.
  • Total Float (TF): By how much time you can delay the task (or activity) so that it does not delay the project finish date or violate schedule constraint. It can be noted as TF.
  • Free Float (FF): By how much time you can delay the task (or activity) so that it does not delay the subsequent task(s) or the successor task(s). It can be noted as FF.

For critical tasks, Total Float can be “0” or can be “Negative”. Critical tasks will have Free Float as “0”. To understand it with an example, you can refer this post:

Primavera P6 - Critical Path is Not Always The Longest Path

Taking one of the figures from the above article, we have two critical paths shown, which are highlighted in read in the graphical side of the Schedule Layout.



 Critical Path 1: Start – Activity A – Activity B = 5 days 
Critical Path 2: Start – Activity E – Activity – Finish = 8 days

As you can see Activity A is not on the longest path (the path is only of 5 days duration), but still it is highlighted as a critical activity. Because this activity has negative total float of value “-1 day”. Of course, the other two activities – Activity E and F are critical activities, because their TF values are zeroes. 

References: 
1. “6.6.2.2: Critical Path Method” from PMBOK Guide 5th Edition.
2. “6.7.2 Critical Path Method (CPM)” from Book - I Want To Be A PMP by Satya Narayan Dash.


Brief Profile: Manas Das, Project Manager Infosys Technologies
Manas Das has 12+ years of work experience and is playing a Project Manager role for retail portfolios in NA geography for Enterprise Application Services.



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