Undoubtedly, Critical Path Method (CPM) is one of the most popular concepts in the Project Management world. CPM is used extensively by project management practitioners around the world for developing project schedules.
Questions related to Critical path are very frequently seen the PMP exam, so this topic is also important from the examination point of view. So, let’s move in…
For understanding the concept of critical path, first, you should understand what a Project Schedule Network Diagram is.
Project Schedule Network Diagram
Project Schedule Network Diagram is a graphical representation of logical relationships (dependencies) among Project Schedule activities. Schedule Network Diagram is the key output of “Sequence Activities” process, which intends to create a logical sequence in which the project activities should be completed in order to obtain the greatest efficiency.
The dependencies between activities in a Network Diagram is called Network Logic.
The above figure is a Project Schedule Network diagram. A, B, C, D, E are the five different activities in this project. For the completion of the project, you should complete all the five activities.
Now, In the diagram, from START to END there are multiple paths. Each of these paths is a sequence of activities with their respective activity dependencies. Each of these paths is called a Network Path.
Therefore, In the above diagram, there are 3 network paths
- START – A – B – END
- START – C – END
- START – D – E – END
Time is one of the major constraints of any project. In fact, it is one of the Triple Constraints (Scope, Schedule, Cost). So every Project Manager looks to reduce the amount of time the project takes. Further, in order to estimate the resources and costs required for the project, we need to know the shortest possible duration required to complete the project. This is where the concept of Critical Path comes in.
Critical Path
Critical path is defined as the longest duration path in the network diagram from START to END that determines the shortest possible duration to complete the project.
This seems confusing, isn’t it ??
There are 2 aspects
- Longest duration path in network diagram
- Shortest Possible duration to complete the project
These two seem to be opposite right? how come these two are equal ??
Don’t worry.. this confusion is common.. let me clear this with our above example.
We already saw the 3 network paths, now let us calculate the duration’s of these paths
- START – A – B – END —– > 50 days
- START – C – END ———-> 30 days
- START – D – E – END ——> 70 days
One thing that should be clearly understood is, Finishing the project does not mean moving from START to END. Finishing the project involves completing all the scheduled activities.. so in the above diagram.. finishing the project involves completing A, B, C, D, & E activities.
Now, amount of duration that all the 5 activities take when executed one by one is the sum of their duration’s i.e… 10+40+30+20+50 = 150 days. But based on logical relationships, you can start A, C, & D activities at the same time, In 10 days, A is completed, and B starts immediately and it completes in the next 4o days. C is completed in the first 30 days. D is completed in the first 20 days, and E starts immediately. So, in the first 50 days, A, B, C, D are completed and E is in progress. E takes 20 more days and completes on the 70th day. Thus all activities are completed in 70 days, so we can say the project is completed in 70 days. If you can see, this is the shortest possible duration to complete all the activities (i.e.. the entire project).
Further, 70 days is the duration of the 3rd network path. It is the network path with the longest duration. Thus, in order to find the shortest possible duration a project will take, just check the longest network path in the network diagram. This longest network path in a network diagram is called Critical Path.
Thus, we can conclude that Critical Path is a network path that represents the longest duration path through a project, and it determines the shortest possible duration for the project.
One important aspect of Critical path is, the Total Float of Critical Path is ZERO ..i.e… the float of any activity on a critical path is Zero.
Now that we have seen what a critical path is, we shall now see what a Critical Path Method (CPM) technique is.
Critical Path Method (CPM)
Critical Path Method (CPM) is a Schedule Network analysis technique, used on both Develop Schedule and Control Schedule processes, to estimate the minimum project duration and to determine the amount of schedule flexibility available on the network paths.
Using this technique, the early start date, Early finish date, Late start date, and late finish date of all the activities are calculated by performing a forward pass and backward pass analysis through the schedule network.
Also check Critical Chain Method (CCM).