The critical path is the longest sequence of activities in a project schedule that is required to be completed on time in order to make the project completion also on time.
Activities in the Critical Path may not be initiated until their predecessors have been completed. So, if a delay occurs in one of the activities included in the Critical Path, this will result in a Project delay.
This concept is the foundation of the Critical Path Method (CPM), the famous algorithm for project scheduling developed in the late 1950s.
- 1 How to calculate the Critical Path
- 1.1 Specify the individual activities
- 1.2 Determine the sequence of those activities
- 1.3 Draw a network diagram
- 1.4 Estimate the completion time for each activity
- 1.5 Identify the critical path (the longest path through the network)
- 1.6 Update the CPM diagram as the project progresses
- 1.7 Use it as a decision making tool
How to calculate the Critical Path
The elements required for identifying the Critical Path are the following:
- The WBS (Work Breakdown Structure), that is, the full list of activities included in your project
- The duration of each of these activities
- The dependencies involved in your project activities
- Milestones and deliverable dates (logical end point)
Once you have these elements, proceed to the following 7 easy steps:
Specify the individual activities
As I mentioned before, the starting point is the full list of activities included in your WBS (in fact the WBS is the “cornerstone” of Project management).
Determine the sequence of those activities
There are activities that can’t start until others have been completed. Others must be initiated in parallel. Others require resources necessary also for other activities. All these requirements result in a complex sequence of activities.
Draw a network diagram
Drawing the CPM diagram is a critical step. There are two approaches here:
- Activity on node (AON): this is the original approach (as it was originally developed).
- Activities on the arcs (the PERT chart approach): in this case milestones are the nodes, and activities with letters A,B,C, etc are in the arrows.
Estimate the completion time for each activity
Activity estimates may be obtained by several methods that were covered in the Project Estimation Professional™ (PEP™) course. Some of them are Expert Judgement, Delphi method, and PERT (optimistic estimate, pessimistic estimate, and most likely estimate).
Identify the critical path (the longest path through the network)
This is the key. We are looking for the longest duration path through the network. Since this is not simple to visualize in complex diagrams, for each activity we make four calculations:
- Earliest Start (ES) time: this is the earliest time an activity can start once the previous dependent activities are over.
- Earliest Finish (EF) time: this is the ES time plus the activity duration.
- Latest Finish (LF) time: this is the latest time an activity can finish without delaying the project.
- Latest Start (LS) time: this is the LF time minus the activity duration.
The Critical Path is the path in your project network where activities have LS=ES and LF=EF.
Update the CPM diagram as the project progresses
Once the Critical Path is calculated, don’t forget to update it whenever there’s new information that may impact in your project network diagram (new activity progress status reports, new estimates based on further knowledge, experience, risks, etc.).
Use it as a decision making tool
The project Manager must always report project status by making special focus on the activities involved in the Critical Path. The picture of the critical path is the key tool for decision making in a project, for both the Project Manager and also Senior Management. The Project Steering Committee and CCB (Change Control Board) require this valuable information in order to diagnose how an individual activity delay may impact on the overall project and therefore make better decisions.
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