PERT and CPM: What are the Differences and How to Use Them Together


Are you a project manager just getting started in the industry or an experienced one with several large-scale projects under your belt? Whatever the case, it’s best to keep yourself abreast of the latest project management tools to keep your project planning and execution skills sharp.

In this blog, we discuss two project management techniques, PERT and CPM, that will increase your project’s efficiency and planning. We dive into what these tools mean, how to use them and why you should use both together.

What is PERT?

PERT, or Project Evaluations Review Technique, in project management, is a work planning process through which managers can identify how long it takes to complete a task. It is a statistical tool that visually charts out all required tasks that must be completed within a specific project. Through PERT, you can also identify the minimum time necessary to complete a task.

PERT was developed by the US Navy in 1958 to manage their special submarine missile program. However, the tool is frequently used by project management teams to simplify and reduce the risk associated with projects.

What are the Benefits of Using PERT?

Managers need tools to help them manage a project and PERT is one of those tools. Other benefits of using PERT are:

  • Maximises resource utilisation.
  • Reduces project risks because managers account for all necessary activities for project completion.
  • Allows managers to estimate a project end date by taking into account the overall task count.
  • Highlights interdependencies between resources.
  • Ensure accountability as every member will be able to see what is assigned to them and others.

But, probably, the best part of PERT is that managers can use it even if they have minimal project schedule data. Lastly, PERT is a great decision making tool for managers as it helps them evaluate where and which resources are needed for current and future projects.

How to Calculate PERT?

The focus of PERT methodology is on time — if time is controlled, project costs will be minimised as well.

To use PERT you need to:

  1. List all the activities
  2. Identify project milestones
  3. Determine activity sequence
  4. Draw a chart/network where activities are arrows and milestones are circles
  5. Estimate activity duration

PERT methodology uses three points to calculate a weighted time average for events in a project:

  • Optimistic (O) - the “best-case” scenario or the shortest time required for a task. This has no risks.
  • Most Likely (M) - account for some risks and delays in calculating time.
  • Pessimistic (P) - This is the “worst-case” estimate and therefore projects the longest duration to complete the work.

What is CPM?

Critical Path Mode (CPM) is a step-by-step project management technique that helps identify the longest sequence of activities that must be completed for a project to be considered “done.” A manager can identify the domino effect that will occur if one task gets delayed by using CPM.

CPM, like PERT, was developed in the late 1950s, as a technique to reduce scheduling inefficiencies and lower project costs. CPM gained popularity in the project management field because it allows teams to break down large projects into bite-sized, actionable steps.

Advantages of Using CPM

Some reasons why you should consider using CPM technique include:

  • CPM minimises the number of bottlenecks in a project.
  • The diagram highlights interdependencies in the work scope, so managers can prioritise tasks. This process also helps identify which activities cannot be completed side-by-side.
  • The tool allows managers to compare time allotted to activities versus actual time taken.
  • CPM also provides a clear communication process between all stakeholders.
How to Calculate CPM

To be able to take advantage of CPM, you need to know how to perform it. Use our step-by-step guide on calculating CPM for your next project.

  1. List all specific activities related to the project.
  2. Identify the interdependencies amongst tasks. What tasks have an immediate predecessor? This will help to generate an activity sequence — what tasks have to be completed first and which can run simultaneously.
  3. Draw out a network diagram with arrows that visualises the progress amongst tasks.
  4. Calculate the time required for each task. You can refer to previous project data, industry standards, or even your own experience to estimate time.
  5. Use the forward pass and backward pass methodologies to calculate the earliest start (ES) time and the latest completion time (LC).
  6. Highlight the critical path. The critical path is the longest time between when the first task starts to the completion of the last task.
  7. Calculate float. Float or slack refers to the amount of flexibility you have within a project. These tasks can get delayed without impacting the final project delivery.

Differences Between PERT and CPM

While both PERT and CPM are techniques that can lead to effective scheduling and project management, they aren’t the same. We’ve highlighted the differences between the tool metrics in the following table.

Type of Tool Visual Tool Statistical Tool
What it Controls Controls time management Controls costs and time
Best Use Scenario Ideal for research and product development projects Used with tasks are non-repetitive Ideal for construction or manufacturing processes Used with tasks are repetitive
Methodology Uses a three-point methodology to estimate time PERT is calculated based on weighted time for events Uses one single methodology to estimate time and cost CPM is focused on activities and highlights interdepencies

How to Use PERT and CPM Together

What should you use? PERT or CPM? The reality is that PERT and CPM are complementary tools. If you want the best results and most accurate planning for your project, you should know how to use them together.

Here’s an example of how you can use both together. Suppose you have to construct your home. Using PERT, outline the overall project path. Now, if you need the house sooner than expected, use the CPM technique to identify the critical path and reduce the overall time required for the project. In this case, you might realise that the civil work is part of the critical path and is possibly taking the most time to complete. With this knowledge, you can reduce the time required for civil work by speaking to the contractors or increasing manpower.

Upgrade Your Project Planning with eRS — Resource Planning Software

Using templates or Excel to perform CPM and PERT analysis can be time-consuming and cumbersome. Work planning and resource planning tools can make this step faster. eRS is an end-to-end resource planning software that will aid you in project planning and execution.

eRS can help you identify the right resources, build capacity plans, and identify the best and most cost-effective process for global projects. Our software is used by over 500 customers and we would love to be your trusted resource planning partner. To stay ahead of the pack, start our no-commitment, free 14-day trial today.

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