PERT and CPM approaches are staples of project management. Combined, these techniques manage, control, and execute projects of varying lengths. Project managers use these techniques to break up complex projects into manageable task sequences and estimate their duration, cost, and dependencies.
Despite their complementary nature, they follow different trajectories and orientations. PERT is a probability chart that focuses on project planning, whereas CPM is a statistical approach responsible for determining the most significant activity in the project.
This article attempts to identify their character and how it differs. It further explores the use cases and comparison of these two project management approaches. Let’s give it a read.
In this article
Part 1. PERT Vs CPM
PERT and CPM charts are project management tools for planning and controlling a project. Having a similar visual layout, these techniques are complementary. Let us discuss how they differ and collaborate to execute a successful project.
| PERT | CPM |
| Abbreviation | Abbreviation |
| Project Evaluation and Review Technique | Critical Path Method |
| Definition | Definition |
| A project management tool used to plan and schedule projects. It estimates a project’s duration and dependencies. | A project management tool used to identify the longest and most critical task sequences and estimate their cost. |
| Orientation | Orientation |
| Event-oriented approach | Activity-oriented approach |
| Objective | Objective |
| The PERT approach focuses on estimating the project duration and deadlines. | The CPM approach focuses on the time-cost of the project’s critical task sequence. |
| Technique | Technique |
| It follows a probability model suitable for research and development projects. | It follows a deterministic model suitable for non-research and industrial projects. |
| Time Estimates | Time Estimates |
| It has three time estimates: pessimistic, optimistic, and absolute minimum time. | It has a single-time estimate. |
Part 2. What is PERT Chart?
The Program Evaluation Review Technique, popularly called the PERT chart, is a project management tool. It visually illustrates the project’s schedule by estimating the tasks, duration, and dependencies. The first-ever PERT chart was developed by the US Navy team in the 1950s to manage the Polaris submarine missile program.
Since then, project managers have used PERT charts to determine a project’s critical path, visualize dependencies, and estimate the resources. It further aids them in tracking the project’s progress and preventing bottlenecks. In addition, having a reference PERT diagram boasts team communication, so everyone invests energy in achieving clearly defined strategic goals.
These charts are often mistaken for Gantt charts and CPMs for their similar character. However, PERT diagrams follow a flowchart structure, making them easily understandable. A typical PERT chart has nodes, rectangles, or circles) to represent the tasks connected through vectors (arrows or straight lines).
PERT Example
Here is a typical illustration of a PERT chart. This Program Evaluation Review Technique demonstrates a step-by-step planning project for building construction. The creator has used sequential directional arrows. Labeling on each connecting line suggests an estimated timeline for each green node. So, teams have an idea about each task’s complexity, making it easy to plan an execution strategy.
Pros and Cons of PERT
Let us see why you should and should not use PERT diagrams.
PERT Chart Pros
- The PERT chart follows a breakdown structure for projects. This makes it easy to evaluate the time for tasks with varying difficulty levels.
- The PERT chart integrates information from across the departments. Therefore, it promotes communication and helps the manager assign responsibilities to specific designations.
- Having an in-depth analysis of dependencies clarifies the deadlines. So, you can follow a strategic sequence of tasks within time.
- PERT charts help you identify factual and subjective errors. Hence, with a what-if data display approach, it is easier to see what’s working and prevent bottlenecks during implementation.
PERT Chart Cons
- PERT charts take resources and skills to create, making them time-consuming. Also, the bad data display can defeat the purpose of the chart.
- PERT diagrams require constant revisions and updates as the project progresses. Doing it repeatedly on paper may not be practical.
Part 3. What is CPM?
Critical Path Method, or CPM, is a project management technique used to identify the most important task in a project. It recognizes the longest sequence of activities, the critical path essential for the project’s success. Also, these events must be completed in a sequence. For instance, you cannot construct a building’s wall before the foundation.
Having an allocated critical path can help project managers improve future planning, manage resources, and prevent setbacks or delays. Moreover, finding a critical path is straightforward. Let us say a project has several task sequences, the activity that takes the longest time is project’s critical path.
The technique was first developed in the 1950s by two mathematicians, Morgan Walker and James Kelley. Kelley attributed the term “critical path” to Booz Allen, who developed the PERT technique during his US Navy submarine design project. The CPM method was developed in an attempt to reduce the cost of a plant restart.
CPM Example
This block diagram of the CPM network depicts how a project is scheduled using the length of its task sequences. It uses different colored arrows to differentiate the activities within the project. Also, the creator has labeled the connectors and included legends in the top-right corner. This saves you from the struggle of attempting to identify the critical path and its duration.
Pros and Cons of CPM
Now, let us break down the advantages and disadvantages of the critical path method.
CPM Pros
- The CPM approach helps project managers identify the parallel activities in a project and figure out which task sequence is the most critical for the project's success.
- With CPM, it is easier to create efficient human networks to work on multiple task sequences simultaneously.
- The CPM chart aids managers in managing project expenses and allocating resources effectively.
- The critical path method chart makes it possible to determine the duration for multiple task sequences and the start, end, slack, and float time for individual tasks.
CPM Cons
- CPM becomes challenging and messy in big and lengthy projects.
- An ill-defined critical path can prolong the project. Not only this, but calculating the critical path is challenging and takes enormous resources like skill, mathematical assistance, and time.
Part 4. Key Differences Between PERT and CPM
Once we know their individual character, the next step is understanding the key differences between PERT and CPM techniques.
Purpose
The apparent PERT-CPM difference lies in their objectives. PERT charts are usually based on planning and scheduling the projects and their total deadline. Quite the opposite, CPM charts focus on the cost and resources of individual tasks in a project. In essence, PERT is about time estimates, and CPM is managing the cost within a reasonable duration.
Orientation in Managing Projects
Project orientation is another area of difference between PERT and CPM charts. A CPM is an activity-oriented approach, meaning that it highlights what’s important. Hence, the manager is equipped to allocate more resources and time to a priority task sequence. However, PERT is an event-oriented technique that focuses on milestones, task durations, and deliverables. It usually results from a critical path.
Use Cases
PERT is a planning and scheduling technique primarily designed for research and development projects. It helps managers visualize the developmental stages and estimate the time for each phase. In contrast, the deterministic model of CPM works best for non-research projects like building construction.
Time Estimates
One of the important differences between both approaches is their time estimates. For instance, CPM has a single time estimate that identifies the critical path and considers delays. In contrast, PERT is more precise and includes three time estimates: pessimistic, optimistic, and absolute minimum time.
Critical Activities
As the name indicates, CPM is a popular statistical approach to finding the longest task sequence in a project. Simply put, it identifies the most important project activity essential for its success. In contrast, the PERT chart attempts to determine the optimal timeframe and schedule of the tasks in a project. Hence, you can integrate a CPM technique in an already-made PERT chart.
Conclusion
Project managers use PERT and CPM together in managing and executing a successful project. Despite following a similar visual pattern, these project management tools have distinct characters and orientations.
A block diagram of CPM evaluates the project’s most critical task sequences, whereas the PERT approach focuses on managing deadlines and estimating targets. Most industries use them in collaboration and integrate a CPM evaluation in already-made PERT charts.
However, making these estimates manually is hectic and takes forever. What’s better is to use advanced mathematical or diagramming software like EdrawMax. This diagramming software has premade CPM and PERT charts that may help you kickstart your project management journey.
EdrawMax Desktop
Edraw.AI
