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The critical path method (CPM) is a method of project management scheduling that uses a mathematical algorithm to derive a logical and efficient order of activities and events, based on the premise that preceding activities determine the course of a project. The adoption of CPM helps save scheduling time and cost. CPM has much in common with PERT or Project Evaluation and Review Technique, with the major difference being that CPM adopts a deterministic approach to estimation of time with no probability whereas PERT allows for variance and probabilities.
A critical path is the path through the series of events or activities that take the most time to reach and complete the final activity.
The length of the critical path determines the duration of the project. Speeding up the activities in the critical path helps complete the project faster, and delays in completion of the activities in the critical path cause the project to suffer from time overrun. An activity outside the critical path does not affect the project completion date if completed faster than scheduled. An activity outside the critical path also does not affect the project completion date even if held up, provided the hold up does not exceed the time it takes to finish the critical path activities.
Project managers try to complete projects faster by “crashing the critical path” or shortening the duration of critical path by adding more resources.
Image Credit: flickr.com/Wouter Kiel
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Critical Path Process
Deriving the critical path involves
- Making a list of all activities in the project, usually categorized within a work breakdown structure.
- Noting down the duration of each activity.
- Denoting the dependencies between the activities. The three major types of dependencies are causal, where the start of one activity is dependent on the completion of the preceding activity, resource, where an activity is limited by availability of resources, and discretionary, or optional sequence preferences that reflect organizational preferences or any special considerations
- identifying the activities with causal dependency for the purpose of critical path calculation. The time taken to complete such “critical” activities determines the length of the critical path. Activities without casual dependencies usually have “float” and can be scheduled parallel to critical path activities to save time
In large complex projects, there can be more than one critical path, or multiple critical paths, This occurs when the critical activities are grouped in more than one sequence. Such multiple critical paths take the same amount of time to complete.
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Critical Paths in Sub-Projects
Large projects are usually broken up into sub phases, and in such cases, each independent sub phase has its own critical path. This is the shortest time to complete the activities contained in the phase, starting from the first activity to the activity without any successor. In such cases, the timely completion of project depends on the timely execution of all multiple critical paths.
Most project management software such as MS Project allows viewing multiple critical paths for each independent network or series of tasks.
The presence of CPM multiple critical paths notwithstanding, the earliest finish date of the project remains the same.
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- Kelley, James. "Critical Path Planning and Scheduling: Mathematical Basis." Operations Research, Vol. 9(3), May-June, 1961.
- Hugg, Bob. "PERT--Key Terms, Concepts and Definitions." http://krypton.mnsu.edu/~tony/courses/609/PERT/defs.html. Retrieved 30 November 2010
- “Managing Your Projects Critical Path.” http://office.microsoft.com/en-us/project-help/managing-your-project-s-critical-path-HA001021173.aspx#BM#5. Retrieved 30 November 2010