The Critical Path Method (CPM) is a simple but powerful technique for analyzing, planning, and scheduling large, complex projects. It is used to determine a project’s critical path—the longest sequence of tasks that must be finished for the entire project to be complete.
CPM, also known as Critical Path Analysis (CPA), identifies dependencies between tasks, and shows which tasks are critical to a project.
What is a critical task?
A task is defined as critical if delaying it will slow down the completion of the entire project. For example, when building a house, laying the foundation is a critical task because it must be finished before you can complete other tasks like building the walls and adding the roof.
How is CPM used in project management?
Project managers (PMs) use CPM to find the best way to schedule all the tasks in a project so that they can be completed as quickly and cheaply as possible. It helps them to prioritize tasks, anticipate bottlenecks, and identify task dependencies, resource constraints, and project risks.
By revealing the critical path through the project, the method helps PMs to accurately estimate the project’s total duration and create a realistic project schedule. This is important as projects which overrun can result in additional costs and stress and potentially to a loss of credibility for the project team.
How does CPM work?
Each project is broken down into a collection of individual work tasks (activities) which must be completed in a specific sequence. A type of logic diagram called an Activity-on-Node (AON) diagram is used to express the project’s sequence (show the order in which the activities must be completed).
In the diagram, each activity is represented by a ‘node’ (a circle or rectangle containing the title of the activity). The nodes are linked by arrows which show the direction of workflow and the relationships between activities.
In the following example, the activity at the base of the arrow (Activity A) must be 100% completed before work can start on the activity to which the arrow is pointing (Activity B). This finish-to-start relationship is indicated by the arrow exiting the right side of the Activity A node (the finish side) and entering the left side of the Activity B node (the start side).
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Once all the activities have been arranged in a logical order and the relationships are set, the calculation process begins.
What are the four stages of critical path calculation?
The Forward Pass
In this opening phase, the Early Start (ES) and Early Finish (EF) values for each activity are calculated along with the overall project duration.
The ES value = the earliest date on which an activity can start
The EF value = the earliest date on which an activity can finish
The ES value is placed in the top left of the node and the EF value is placed in the top right. The number of days needed to complete each activity is written inside the node underneath the activity.
The ES for the very first activity in the project is always a zero. The ES value of every other activity will always be the highest EF value of the tasks that precede it. An activity’s EF value = its ES value + duration.
In the example below, Activity E cannot begin before Activity B and Activity C are completed. Since Activity B has an EF of 11 and Activity C has an EF of 11, Activity E’s ES value will be 11. Activity E will take two days to complete, so its EF value (11 + 2) will be 13.
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In the example above, the EF value of the final activity is 16, meaning that the project will take at least 16 days to finish.
The Backward Pass
In the second phase, the Late Start (LS) and Late Finish (LF) values are calculated.
The LS value = LF – Duration
The LF value = the lowest LS value from the immediate successor node(s)
This calculation is made backward, starting from the final activity (Activity F in this example). The LS value is placed in the bottom left of the node and the LF value is placed in the node’s bottom right.
The process starts with adding in the LF value of the final activity, which is always that activity’s EF value. Calculating the backward pass always begins with ‘dropping’ that final EF down into the LF slot.
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Float Calculations
Float values show how much each activity can be delayed before it delays the activities that succeed it or the planned project completion date.
The Total Float value is the maximum number of days the activity can be delayed without delaying the project completion date. It is written above the node on the left.
The Free Float value is the maximum number of days an activity can be delayed without delaying any succeeding activities. It is written above the node on the right.
Total Float = LS – ES (also LF – EF)
Free Float = Lowest ED of successors – EF
In the example below, Activity C has three days of float before the start of Activity E will be delayed.
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Calculating the Critical Path
Calculating the float values will reveal the critical path as the string of activities that do not possess any days of float (critical activities). In the example above, the critical path is:
Activity A -> Activity B -> Activity D -> Activity F.
The total duration of all the critical activities (16 days) is equal to the overall project duration.
It’s important to note that a schedule can have more than one string of activities that make up the critical path. However, the critical path must be continuous from the first activity in the schedule to the final activity.
Why must non-critical tasks be monitored?
Non-critical tasks are not dependent upon the completion of other tasks so they can be executed before or after a certain project stage has been completed. However, as these tasks still need to be done for the project to be finished, forgetting one will push back the completion date of the whole project.
For CPM to work effectively, your schedule must be continually updated throughout the project to reflect any changes. Using project management software will make it quicker and easier to plot and monitor your critical pathway and adjust your schedule when necessary.
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