CPM (Critical Path Method)
What is CPM in Project Management and how is it used?
The Critical Path Method (CPM) is a tool that helps in easily planning and scheduling the different activities in a project. It is a modelling technique which highlights the tasks that can directly impact the timelines of a project.
This method can be effectively used in every project that can be broken down into smaller activities. Some of these activities could be independent from others and these could be performed in any order. Whereas other activities might have a dependency on each other and there might be a particular sequence for completing these tasks.
This is the main idea behind the Critical Path Method in Project Management. It provides a pictorial representation of the list of tasks in a project and puts special focus on the activities that fall on the Critical Path.
Critical Path
The foundation of the CPM is the concept of Critical Path in the project. This path indicates the particular sequence of tasks that would take the longest total time from the start to the end of the project.
The time it takes to complete the activities along the Critical Path would be the minimum time required to finish the project. So, if there is a delay in completing any of these tasks, then the timelines of the entire project will get extended.
If the Project Manager wants to speed up the project delivery, then the priority should be to reduce the time required for these activities. Similarly, any delay in these activities has to be avoided because it can delay the entire project.
Difference between CPM and PERT
The Program Evaluation and Review Technique (PERT) is used to highlight all the activities in a project and the order for completing these activities. The CPM technique can be thought of as a component of the larger PERT analysis.
When using PERT, it is important to build a Network Diagram that shows the complete list of tasks in the project. After that, the Critical Path is identified and special attention is given to the activities that fall on this path.
The Network Diagram also indicates the time required to complete each task and this is used for properly allocating the resources at different stages. So, in actual projects, PERT and CPM are used together for project planning purposes.
CPM Network Diagram
The first step when designing a CPM chart is to identify the list of activities in a project. After that, the scope of each task and the time required to complete the task has to be analyzed. In addition, it is important to determine the sequence of the activities by finding any cross dependencies between them.
Once all this data has been collected, the next step is to construct the Network Diagram. The activities are represented by nodes in the diagram. Finally, the user can identify the Critical Path by finding the longest sequence of activities, from the start to the finish.
The activities that lie on the Critical Path are monitored carefully because these can impact the project timelines. As the project progresses, changes can be made to the Network Diagram.
Critical Path Method example
A sample CPM chart for a small project can be seen in the below image. The time estimate for each task and other data in the image is just used as an example. The nodes (A, B, C, D etc.) represent the activities in the project.
In any branch, a new activity can only be started after the immediately previous activity has been completed. In the below diagram, the longest path to get from the Start to the End is by performing the tasks in the following order:
A -> B -> D -> E
It would take a total of 2 + 3 + 5 + 3 = 13 days to complete all the activities on this path. And since these activities have to be done in a sequential manner, the minimum time required to complete the project will also be 13 days.
All the other branches will take less than 13 days to complete. For example, performing the activities A -> B -> C will take 2 + 3 + 5 = 10 days. Whereas completing the tasks F -> G will take 7 days.
CPM analysis
As it can be seen from the above image, this type of chart is very flexible and it can be used in different industries. It clearly depicts a list of activities and helps in organizing the flow of the project. When using the CPM, there are 4 important parameters which have to be defined for each activity:
- Earliest Start date
- Latest Start Date
- Earliest End Date
- Latest End Date
Example: In the above sample diagram, there are a total of 7 activities in the project. The critical path has already been identified as A -> B -> D -> E and it would take 13 days to complete all the tasks in the Critical Path.
Let us assume that the management wants to complete the project in the shortest possible time. Suppose that the start of the project happens with the beginning of the first activity and the end happens with the completion of the last activity. So, the project starts on Day 1 and it will end on Day 13.
Based on this information, the possible dates for each task have been shown in the below table.
|
Activity |
Earliest Start Date |
Latest Start Date |
Earliest End Date |
Latest End Date |
|
A |
Day 1 |
Day 1 |
Day 2 |
Day 2 |
|
B |
Day 3 |
Day 3 |
Day 5 |
Day 5 |
|
C |
Day 6 |
Day 9 |
Day 10 |
Day 13 |
|
D |
Day 6 |
Day 6 |
Day 10 |
Day 10 |
|
E |
Day 11 |
Day 11 |
Day 13 |
Day 13 |
|
F |
Day 1 |
Day 7 |
Day 1 |
Day 7 |
|
G |
Day 2 |
Day 8 |
Day 7 |
Day 13 |
Slack Time
The Slack Time for an activity is defined as the maximum time that the Earliest Start Date or the Earliest End Date could be delayed, without delaying the entire project. The Slack can be simply measured by taking the difference of the Earliest Start and Latest Start or by taking the difference of Earliest End and Latest End.
In CPM method, it is possible to calculate the Slack Time for each individual activity. Example: In the table used for our above CPM diagram, the Slack Time for activity C is 3 days, for activity F it is 6 days and for activity G it is 6 days.
It should make inherent sense that there can be no Slack Time in the activities that fall on the Critical Path. This is because any Slack in these activities will delay the entire project. It can even be noticed for activities A, B, D, E in the table that the Earliest Start and Latest Start is same. Also, the Earliest End and Latest End for these 4 activities is also same.
Advantages and Disadvantages of CPM
This is an efficient way for communicating the timelines and the most important steps in a project. It also helps in better planning because the project timelines can be shortened by allocating more resources to the tasks that require more time. On a high level, the benefits of the Critical Path Method are as follows:
- It has a flexible design and it can be used in a wide range of projects and industries
- Lays out a list of all the tasks in a project and provides the time required for each task
- Clearly highlights the most important activities for timely completion of the project
- Indicates the minimum time required for completing the entire project
- It is better than a Gantt chart because it displays the cross-dependencies of different tasks
- Highlights the activities which have to be performed sequentially and the activities that could be done independently
Problems with Critical Path Method
When the Network Diagram is being built, some assumptions and estimations have to be made about the time required for each task. These could be Deterministic Estimates or Probabilistic Estimates. So, the accuracy of these estimations will govern the difference between the actual timelines and the predictions.
Moreover, a Network Diagram can become very complex in case the project is large and if it has multiple tasks. The users might need to use some project planning software to build this diagram and to make any required changes in the middle of the project
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