Historical Development of Network Analysis: History of PERT and CPM
Introduction — Network Analysis is an important part of Operations Research used for planning, scheduling, and controlling projects. It is especially useful when a project contains many activities that are related to one another.
The modern development of project network techniques took place mainly during the late 1950s. Two important techniques emerged during this period: PERT (Program Evaluation and Review Technique) and CPM (Critical Path Method).
PERT and CPM were developed independently by different groups for different project environments. PERT was developed in connection with the U.S. Navy's Polaris program, where activity times were uncertain. CPM was developed through the work of DuPont and Remington Rand for industrial project scheduling, where activity times could generally be treated as known or reasonably predictable.
Although PERT and CPM had different origins, both became important tools for project planning, scheduling, and control.
Historical Background of Network Analysis
Before PERT and CPM were developed, project managers used several planning and scheduling methods, including Gantt charts, flow diagrams, milestone methods, and other scheduling techniques.
These methods were useful, but large and complex projects created new difficulties. Managers needed a better way to show the logical relationships among activities, identify activities that could affect the project completion date, and study the effect of changes in the project schedule.
PERT and CPM developed in this environment during the late 1950s. Therefore, they should be viewed as an important development in project scheduling rather than as techniques that suddenly replaced all earlier methods.
Historical Development of PERT
Origin of PERT
PERT was developed in 1958 in connection with the Polaris Fleet Ballistic Missile program of the U.S. Navy.
The development involved the U.S. Navy's Special Projects Office, Booz, Allen and Hamilton, and the Lockheed Missile Systems Division. The Polaris program was a large research and development effort involving many interrelated activities and organizations.
One of the major difficulties was that the exact duration of many activities could not be known in advance. This created a need for a project scheduling technique capable of dealing with uncertainty in activity times.
The resulting technique became known as PERT.
The original technical paper published in Operations Research in 1959 described PERT as a method for measuring and controlling progress in the Polaris Fleet Ballistic Missile program. It represented the research and development program as a network of interrelated events and incorporated uncertainty into activity-time estimates.
Meaning of PERT
PERT stands for:
Program Evaluation and Review Technique
An interesting historical detail is that the original project documentation referred to PERT as “Program Evaluation Research Task.” The technique later became widely known as Program Evaluation and Review Technique.
PERT was designed for situations in which activity durations were uncertain. It therefore became closely associated with research, development, and other projects where reliable time estimates were difficult to obtain.
Why Was PERT Developed?
The Polaris program involved a large number of interdependent activities. Many of these activities were associated with research and development work, so their completion times were not known with certainty.
PERT helped management to:
- Represent the logical relationships among project activities.
- Estimate project completion time under uncertainty.
- Evaluate the progress of the project.
- Study the effect of changes in the project schedule.
- Assess the probability associated with achieving important project targets.
The original PERT work specifically emphasized integrated and quantitative evaluation of project progress, plans, schedules, and proposed changes.
Historical Development of CPM
Origin of CPM
At approximately the same time that PERT was being developed, another project scheduling technique was being developed independently in the industrial sector.
CPM (Critical Path Method) was developed through the work of Morgan R. Walker of DuPont and James E. Kelley Jr. of Remington Rand during 1956–1957.
DuPont was interested in improving the planning and scheduling of engineering and maintenance projects. Walker and Kelley worked on developing a network-based method that could represent the logical relationships among project activities and help management analyze project duration and cost.
Historical accounts trace the development work to 1956, with important development and testing taking place during 1957.
Main Purpose of CPM
The Critical Path Method was developed to help managers:
- Plan and schedule project activities.
- Identify the critical path.
- Determine the project duration.
- Identify activities that have little or no scheduling flexibility.
- Study the relationship between project time and cost.
The time-cost aspect became an important feature of CPM. It provided a basis for studying whether additional expenditure on activities could reduce the overall project duration. This process is commonly known as project crashing or time-cost trade-off.
Standard OR texts continue to treat project time-cost trade-off and crashing as important parts of CPM.
PERT and CPM Developed Independently
PERT and CPM are sometimes presented together as though they were developed as a single technique. Historically, this is not correct.
They developed independently and in parallel:
PERT
Developed in connection with the U.S. Navy's Polaris program.
CPM
Developed through DuPont and Remington Rand for industrial project scheduling.
Their origins were different, but both used network-based approaches for project planning and scheduling. Historical reviews describe them as parallel developments that grew from earlier project-planning practices.
Timeline of PERT and CPM Development
| Period | Development |
|---|---|
| Before the 1950s | Project managers used methods such as Gantt charts, flow diagrams, milestone methods, and other scheduling techniques. |
| 1956 | DuPont began systematic work on improving project planning and scheduling. |
| 1956–1957 | Morgan R. Walker and James E. Kelley Jr. developed the ideas that became CPM. |
| 1957 | CPM was tested in an industrial project environment. |
| 1958 | PERT was developed for the U.S. Navy's Polaris program. |
| 1959 | The early PERT work was described in Operations Research. |
| Late 1950s onward | PERT and CPM spread to other project-planning and scheduling applications. |
The precise development history is more complicated than assigning a single year to each technique, so 1957 for CPM and 1958 for PERT are useful examination dates, while the development process itself extended over a period of time.
Difference Between PERT and CPM
PERT and CPM are closely related network techniques, but their traditional approaches are different.
| Basis | PERT | CPM |
|---|---|---|
| Full Form | Program Evaluation and Review Technique | Critical Path Method |
| Historical Origin | U.S. Navy's Polaris program | DuPont and Remington Rand |
| Development Period | 1958 | Mainly 1956–1957 |
| Activity Time | Traditionally treated as uncertain | Traditionally treated as known or deterministic |
| Main Emphasis | Scheduling under uncertainty | Scheduling and time-cost considerations |
| Time Estimates | Traditionally uses three estimates: optimistic, most likely, and pessimistic | Traditionally uses a single activity-time estimate |
| Typical Early Application | Research and development | Industrial, construction, and maintenance projects |
| Probability Analysis | Important feature | Not a basic feature of traditional CPM |
| Time-Cost Trade-off | Not the original main emphasis | Important feature of CPM |
The distinction above reflects the traditional textbook treatment of PERT and CPM. Modern project management does not always maintain a strict division between “PERT projects” and “CPM projects.” Both techniques have been applied more broadly over time.
PERT and CPM: Are They Used Only for Different Types of Projects?
No.
It is common in textbooks to say:
PERT is suitable for research and development projects, while CPM is suitable for construction and industrial projects.
This statement is useful for understanding their historical origins, but it should not be treated as an absolute rule.
PERT was originally developed for a research and development program in which activity times were uncertain. CPM was developed in an industrial environment where activity times could be treated as relatively known and where time-cost considerations were important.
Today, both PERT and CPM are used in a variety of project-management situations. The more useful distinction is based on the nature of the activity-time information and the purpose of the analysis, rather than simply on the industry in which the project occurs.
Importance of PERT and CPM in Network Analysis
The development of PERT and CPM changed the way large projects could be planned and analyzed.
A network representation allows the project manager to see:
- Which activities must be completed before others can start.
- Which activities can proceed simultaneously.
- How long the project may take.
- Which activities are critical to the project duration.
- Where scheduling flexibility exists.
- How changes in activity duration can affect the project.
- How time and cost can be studied in CPM.
These concepts form the basis of critical path analysis, float calculation, project scheduling, PERT probability analysis, and time-cost trade-off covered in standard Operations Research courses.
Why Is the Historical Development of PERT and CPM Important?
Understanding the history makes the difference between the two techniques easier to remember.
PERT emerged from a project where uncertainty was a major concern.
CPM emerged from an industrial environment where project duration and cost were important management concerns.
Therefore, an easy way to remember their traditional origins is:
PERT → Polaris → Uncertain activity times
CPM → DuPont/Remington Rand → Critical path and time-cost analysis
This is a useful memory aid for students, but the modern applications of both techniques are much broader.
Exam Tip
If the question “Discuss the historical development of PERT and CPM” appears in an examination, do not write only the difference between PERT and CPM.
A good answer should include:
- Brief introduction to Network Analysis.
- Historical development of PERT.
- Polaris program and U.S. Navy.
- Development of CPM by Morgan R. Walker and James E. Kelley Jr.
- The approximate development years.
- Traditional difference between PERT and CPM.
- A short conclusion.
Important dates to remember
1956–1957 → CPM development
1958 → PERT development
1959 → Early PERT technical paper published
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Frequently Asked Questions
Q. When was PERT developed?
PERT was developed in 1958 in connection with the U.S. Navy's Polaris program.
Q. Who developed PERT?
PERT was developed by a team associated with the U.S. Navy's Special Projects Office, Booz, Allen and Hamilton, and Lockheed in connection with the Polaris program. The early technical paper was authored by D. G. Malcolm, J. H. Roseboom, C. E. Clark, and W. Fazar.
Q. When was CPM developed?
The main development of CPM took place during 1956–1957, through the work of Morgan R. Walker of DuPont and James E. Kelley Jr. of Remington Rand.
Q. Who developed CPM?
Morgan R. Walker of DuPont and James E. Kelley Jr. of Remington Rand are credited with the development of CPM.
Q. What is the main historical difference between PERT and CPM?
PERT was developed for the Polaris research and development program, where activity times were uncertain. CPM was developed in an industrial setting and placed greater traditional emphasis on the critical path and time-cost relationships.
Q. Are PERT and CPM still used?
Yes. PERT and CPM remain important project-network techniques and are widely taught as part of Operations Research and project management.
Conclusion
The historical development of Network Analysis is closely associated with the emergence of PERT and CPM during the late 1950s. PERT was developed in 1958 for the U.S. Navy's Polaris program, where uncertainty in activity times was an important problem. CPM was developed independently through the work of Morgan R. Walker of DuPont and James E. Kelley Jr. of Remington Rand during 1956–1957.
Although the two techniques originated in different environments, both provided a systematic way to represent project activities, analyze their relationships, and improve project planning and scheduling. Traditionally, PERT has been associated with uncertain activity times, while CPM has been associated with deterministic activity times and time-cost analysis.
For students, the easiest historical distinction to remember is:
PERT → Polaris → 1958 → uncertainty
CPM → DuPont & Remington Rand → 1956–1957 → critical path and time-cost analysis
References
- Hamdy A. Taha, Operations Research: An Introduction, Pearson.
- Frederick S. Hillier and Gerald J. Lieberman, Introduction to Operations Research, McGraw-Hill.
- J. K. Sharma, Operations Research: Theory and Applications, Macmillan.
- Kanti Swarup, P. K. Gupta and Man Mohan, Operations Research, Sultan Chand & Sons.
- D. G. Malcolm, J. H. Roseboom, C. E. Clark and W. Fazar, “Application of a Technique for Research and Development Program Evaluation,” Operations Research, 1959.
- James E. Kelley Jr. and Morgan R. Walker, historical work on the development of the Critical Path Method.
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