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Difference Between PERT and CPM
CPM stands as Critical Path Method whereas PERT stands as a Project/Programme Evaluation and Review Techniques.
CPM/PERT can be described as a pictorial representation of inter-relationship and sequence of all the required construction activities which constitute the Project.
CPM and PERT are regarded as practical numerical tools of project management, and they facilitate the work of planning, organizing, scheduling, controlling, monitoring and evaluating the Project in the most effective way. The difference between CPM and PERT are as follows:
What is PERT?
PERT or Programme Evaluation and Review Technique is a management tool used for planning, controlling and reviewing a project. The U.S. Navy developed it along with a management consultancy firm for its Polaris Missile Program.
An essential characteristic of any project is its duration. The effectiveness and economy of the Project are often dependent on the project duration.
The total project duration depends on the time taken for each event to occur. To estimate the time required for each event to happen, it requires the planner to have vast experience, with similar.
The estimate of the time duration can be done in two ways. One is the deterministic approach in which the planner with enough knowledge about the Project gives a single estimate of the period, which is sufficiently accurate.
The other method is a probabilistic approach in which precise information is available, hence only the limits within which the duration will lie, is estimated.
PERT follows this second approach or the probabilistic approach towards time durations. Pert is also an event-oriented network method where more importance is given to the achievement of a real milestone rather than the activity itself.
Because of these characteristics PERT was developed and has been used in R and D type projects such as space industry, defence industry etc.
Such projects are of a non-repetitive type or once-through type in which correct time estimates cannot be made.
What is CPM?
The CPM or Critical Path Method was discovered jointly by Dupont and Remington Rand Univac in 1957. This research team was trying to determine how best to reduce the time required to perform routine plant overhaul, maintenance and construction work.
Since the CPM is found to be of much use in the construction industry with applications in the construction of massive structures like bridges, dams, tunnels, high-rise buildings etc.
CPM networks are usually used for the repetitive type of Project, where reasonably accurate estimates of time can be made for the activities of the Project.
The actions of these Projects are characteristically subject to a relatively small amount of variation. That’s why CPM is not suitable for research and development types of projects.
PERT vs CPM
| S.No. | CPM | PERT |
| 1. | It is basically having a DETERMINISTIC approach in the design of the network. | It is basically having a PROBABILISTIC approach in the design of the network. |
| 2. | Only a one-time estimate is required for each activity. | Three-time estimate (in the form of Pessimistic time, Optimistic time and Most likely time) for each activity. |
| 3. | It is built up of an activity-oriented diagram. | It is built up of an event-oriented diagram. |
| 4. | Time and cost both are controlling factors. There is an optimum duration of the project at which the project cost is minimum and this can be obtained by cashing the network. | Time only is the controlling factor and the cost is assumed proportional to the project duration. |
| 5. | Critical events must have zero. | Critical events may be positive/negative depending upon the project scheduled completion time. |
| 6. | It is suitable for moderate time estimation. | It is suitable for high accuracy time estimation. |
| 7. | It has the repeated nature of the job. | It has Non-repeated nature of the job. |
| 8. | Due to certain time boundation, There may be crashing. | Due to the uncertainty of time, there is no chance of crashing. |
| 9. | CPM uses dummy activities for describing the activities sequence. | PERT doesn’t use any dummy activities. |
| 10. | It highly concentrates on time cost trade-off as reducing cost is more significant. | It highly concentrates on time as meeting time targets or estimation of percentage completion is more prominent. |
| 11. | It is recommended for the repetitive nature of work where past experiences are adequately available. | It is recommended for research and development projects where no past experiences are available or where great uncertainty lies in the time estimation. |
PERT is the general case where three times estimates describe the activity time, measure the uncertainty by standard deviation and variance.
In contrast, the CPM is the particular case when all the three-time assessments coincide with one value ( called deterministic time) and the standard deviation and variance disappear.
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