Difference between PERT and CPM | Time, Suitability, Limitation

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In the management of any kind of project, the whole task can be basically divided into 4 phases. They are; project formulation phase, project planning and scheduling phase, project implementation phase, and project termination phase. 

The project implementation phase governs 80%-85% of the project and the remaining others 5%-6% each. However, the project planning and scheduling phase are one of the most important phases in order to run the project smoothly and harmoniously in any profession.

After identification of the resources, objectives, and goals, the project is proceeding with the financial and technical proposal analysis along with the planning of the statement of work and work breakdown structure.

And then the planned works which are to be performed are sequentially needed to assigned with the expected duration of work along with the determination of time to start and finish.

Thus, Program Evaluation &  Review Technique (Notated as PERT)  and Critical Path Method (Notated as CPM are the two of the advanced methods/tools of the project schedule.

And due to the use of network diagrams (diagram represented by arrows for series of work/activities), both are grouped in network analysis. Due to their unique features, the method is suitably adapted as the project’s nature, size, duration, complexity, and requirements.

For example, CPM is an activity-oriented method of scheduling and is suitably adapted for simple and repetitive projects like construction works and engineering.

Whereas, PERT is even oriented method of scheduling and is suitably adapted for new and complex projects like Research and development, developing and launching a new model of airplanes, etc. Let us see the difference between PERT and CPM.

CRITICAL PATH METHOD (CPM)

CPM was developed in 1956 by Margon R. Walker of the DuPont company as a construction company and James E. Kelly Jr. of Remington Rand as Consultants, as a deterministic approach of scheduling method.

CPM is very useful and adopted in Engineering and construction industry. CPM can show the interrelationship between activities along with scheduling of cost, resources, and time.

But it requires an extensive detailed description of the interrelationship of activities that helps in effective project management. Thus identify and interfacing activities required to complete the project is the most difficult task of this method.

This method helps to identify conflicts in resources that may occur due to interrelationship between the work and so the project manager can schedule the work at an inappropriate time, the sequence with optimal resources.

There are two methods of drawing a CPM diagram;

a) Arrow diagram and

b) Precedence diagram.

Although both methods give the same result, the precedence diagrams are mostly adopted for CPM analysis. CPM diagrams are capable of showing sequence and interdependence of activities along with distinctive times of work duration and point of ending and starting.

So in CPM, the duration of each activity is predefined with a distinctive degree of certainty for the project where type and amount of work are determinable. Thus there are start dates, finish dates, and the total time that could possibly take or targeted to finish.

The sole aim of this method is only concerned with finding the critical path in order to find critical time to finish the project in optimal use of resources.

PROGRAM EVALUATION & REVIEW TECHNIQUE (PERT)

PERT was developed by US Navy, with Booz, Allen, &  Hamilton Management consultants. It is the probabilistic approach to scheduling

Unlike in CPM, in some of the projects like Research and Development, Risk management it’s cumbersome to distinctively determine the amount of time duration for every single activity.

The activity may even not happen according to plan and thus the alternative event must be chosen if the planned activity doesn’t occur.

PERT method is devised for such projects along with three types of time duration(optimistic time, pessimistic time, and most likely time) for each activity and the probability & statistics approach for finishing the project.

The method is not much suitable for the engineering approach, however, is very useful In assigning the risk management techniques in the project. PERT is highly recommended for projects with a high degree of uncertainty.

PERT uses the Arrow network diagram whereas CPM commonly use precedence diagram more (but also applicable for CPM). In PERT activity is notated by arrows with a circle at each end of the arrow and circles are called events that represent instant in time.

The circle at front of the activities represents the start of the activity and the circle at end of the arrow represents the finish of the activity. PERT is carried out in backward flow from the deadline date of projects.

Difference between PERT and CPM | Time and Approach, Suitability, Limitation

Difference between PERT and CPM

ParametersCPMPERT
Time and ApproachCPM has one time only and has a deterministic approach. Focus on critical path and time to complete work.PERT has probabilistic approach and has three times to consider. t0 = optimistic time(favorable time) tp = pessimistic time(not favorable time) tm = most likely time texp (average time) = (t0+4tm+tp)/6
OrientationCPM is activity oriented. Thus very useful to establish relationship between the two activities and makes flow of sequence to for critical path of doing such activities in order.PERT is event oriented. Thus helps in predicting of events and their management.
Project parametersIn CPM time and cost are related to each other. Thus CPM focus on both time and cost of projects.PERT focus only on the time of projects. And no relation of coat with time
SuitabilityCPM is suitable for simple and repetitive projects like construction works and engineering where the works are determinable and easy to break down into simple forms. They are suitable for large scale works.PERT is suitable for the new and complex projects like Research and development and manufacturing companies. Extremely suitable for high precision works.
Maintaining and operationCPM easy to operate and maintain.PERT are highly costlier to maintain, required highly skilled manpower.
Network diagramCPM uses both arrow diagram and precedence diagram. But it is recommended to use precedence diagram for easier operation in CPM and tackle dummy activities.PERT uses only arrow diagram.
Special techniquesCrashing is used to reduction of duration of critical path. Uses of dummy activitiesMerge and burst is used in event to deal with activity. No crashing techniques in PERT. No uses of such dummy activities.
LimitationsIt is not suitable for the precise project as it doesn’t use probabilistic and statistical approach. Requires extensive detail of interrelationship of activitiesVery expensive to maintain Is not able to relate the cost of projects.

I hope this article on “Difference between PERT and CPM” remains helpful for you.

Happy Learning – Civil Concept

Contributed by,

Civil Engineer – Rajan Shrestha

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