Critical Path Method (CPM)

Completing a project in time is one of the most important objectives and expectation of the stakeholders. Critical Path Method is used to prepare the optimized project schedule involving all the project activities. 

Critical Path Method helps in:

1. Determining the minimum time in which the project can be completed
2. Determining the sequence of activities which must be completed on time in order to complete the project in time
3. Determining which all tasks can be delayed without delaying the project completion time
4. Determining the Early and Late Start of tasks
5. Tracking project progress with regards to agreed timeline and taking proactive corrective action if the project seems to be getting delayed

The typical steps involved to develop a project schedule using CPM method are as below:

1. Identify the activities for all the work packages from the project’s Work Breakdown Structure (WBS)
2. Sequence all the activities by identifying all the dependencies between the activities. 
3. Develop a Schedule Network Diagram involving all the project activities ensuring that each activity has at least one predecessor and one successor except the first activity which will not have a predecessor and last activity which will not have a successor. 
4. Estimating the duration of each activity in the schedule network diagram.
5. Carrying out the process of “Forward Pass” where in the “Early Start” (ES) and “Early Finish” (EF) for each activity are calculated starting from the beginning of the network diagram.
6. Carrying out the process of “Backward Pass” where in the “Late Finish” (LF) and “Late Start” (LS) for each activity are calculated starting from the end (Finish) of the network diagram.
7. Identifying the “Path” which has the longest duration in the Network Diagram. The longest path will also have the ES and LS and EF and LF of all the activities as same.
8. The longest path is termed as the “Critical Path”. The duration of this path will determine the shortest time taken to complete the project. Any delay on this path delays the project completion time. Hence they are critical from project’s schedule constraint point of view.
9. The “Non-Critical-Path” path duration will be shorter than the “Critical Path” and hence those paths will have flexibility to delay the start of the tasks on them. 
10. The amount of time a task can be delayed on a “non-critical path” is known as “float” or “slack”, which is calculated by taking the difference between “LS-ES” or “LF-EF”.
11. The float on critical path will be Zero to start with and not-critical paths will have a positive float time.
12. There may be more than one critical path in a network. But having more than one critical path increases the risk of falling behind the schedule as there are more number of tasks which if they get delayed, the project will get delayed.

An example of Critical Path analysis is as below:

In the above diagram, the path with longest duration is Start-D-E-F-G-End is the critical path with duration of 17. The other 2 non-critical paths Start-A-B-C-G-End has duration of 10 and hence has a slack or float of 7 and other non-critical path Start-D-H-I-End has a duration of 11 and has a float of 6 days.

Critical Path once identified, the team can further explore if the duration of critical path can be compressed if the need be. Techniques such as crashing (applying more resources on critical path) or Fast Tracking (doing tasks in parallel) are applied. Compressing the critical path helps in compressing the overall project duration, thereby helping to meet the required deadline.