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The Critical Path Method (CPM) Example
Using an activity breakdown and assigning durations and dependencies for those activities leads to the Critical Path Method (CPM) of effort estimation. “Critical path analysis of a project schedule tells us who must wait for what as the project is being developed. It also tells us which activities must be completed on schedule to avoid delay” (Pfleeger and Atlee, 2006, p. 91).
The steps of the CPM of project planning are as follows:
1. Specify the individual activities.
2. Determine the sequence of those activities.
3. Draw a network diagram.
4. Estimate the completion time of each activity.
5. Identify the critical path (longest path through the network)
6. Update the CPM diagram as the project progresses.
Following the steps of CPM development will lead to a diagram similar to that of Figure 1: CPM Diagram as follows:
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This figure illustrates that the critical path for the example project is A=>C=>F and that the projected project completion time is 7 weeks. The critical path and the project completion time are determined by adding the durations of each of the nodes in the diagram. Expert judgment determines the length of each of the activities so, “the accuracy of the prediction is based on the competence, experience, objectivity, and perception of the estimator” (Pfleeger and Atlee, 2006, p. 106). There is no room for error along the critical path because any delay there will cause late delivery of the entire project.
Effective project management requires a reliable method to determine completion times for deliverables or milestones. The CPM is one method for predicting those times without which, project management’s schedules may be highly erroneous. The main advantage of the CPM is the time analysis of dependent activities and the project as a whole. The disadvantage is the reliance on expert judgment to determine the durations of individual activities.
When used in a traditional, waterfall, methodology of system development the CPM is not the best method of effort prediction because each activity leads to the next so all activities are essentially located on the critical path. Agile methods benefit most by the CPM because many activities run in parallel when using an Agile methodology, although the CPM diagram may need modification to account for the many feedback loops. In these instances the CPM helps identify activities that may require more resources to meet the deadlines for deliverables. This would include COTS applications where many copies of the same release will be distributed.
Baker, S., L. (2004). Critical Path Method (CPM) [Electronic version]. University of South Carolina. Available from the Arnold School of Public Health.
NetMBA.com (2007). CPM – Critical path method [Electronic version]. Available from http://www.netmba.com/operations/project/cpm/.
Pfleeger, S., L., and Atlee, J., M. (2006). Chapter 3 planning and managing the project. Software Engineering Theory And Practice (3rd ed.). Upper Saddle River, NJ: Pearson Education, Inc.