12 Key Difference Between PERT and CPM (With Definition & Chart)

Today, virtual meet-ups have shifted the narrative of workplaces from face-to-face discussions and debates to presentations and projects. Project management has evolved lately as an indispensable skill in the corporate world. It offers several opportunities for professionals to explore new careers and fields.

In this article, we will understand the difference between PERT and CPM and how they facilitate project management.

What is project management?

Before we plunge into exploring the difference between PERT and CPM, let's understand what is project management.

Project management may be defined as a systematically carved approach to planning, scheduling, plotting, regulating, and managing the several components of a project that help us achieve the objectives set forth at the outset.

In this way, a project manager plays an important role in conceptualizing and steering a project.

Read about project manager interview questions here: Top 30 Project Manager Interview Questions & Answers You Must Know!

What is PERT and CPM

In this section, we will provide you with a brief overview of PERT and CPM. It will help you to easily differentiate between PERT and CPM.

There are several ways in which we can differentiate between PERT and CPM. We will cover the most crucial aspects of the PERT and CPM difference. So, if you are wondering "What is PERT and CPM?," continue to read.

Let’s begin by looking at PERT and CPM separately.

PERT

The network-based project management approaches (Project) Program Evaluation Review Technique (PERT) and Critical Path Method (CPM), developed in the 1950s, depicts the flow and sequence of activities and events.

PERT is suitable for unpredictable activities in which the time required to accomplish certain tasks is unknown. In this way, PERT is used for organizing, scheduling, and integrating different tasks related to a project. In other words, PERT assists in providing a blueprint for a project and aids in project evaluation.

PERT is a technique for evaluating, coordinating, executing, and reviewing a project that uses project activity duration as a key variable. It enables a project manager to figure out the minimum time and additional cost required to complete an entire project or one of its subset project activities.

First and foremost, the project is separated into activities and events using this method. Following that, the correct sequence is determined, and a network is built. The critical path (the longest path linking all the events with maximum time) is then found by the control of time required for each activity.

CPM

The difference between PERT and CPM is seen in the nature of projects. CPM is best for projects that are repeated. Unlike PERT, projects with a defined timeline use CPM. In this way, CPM is used for determining how much time a project will take and offers a reasonable time estimate.

The Critical Path Method is known as such because to save time and eliminate queue building in the process, it distinguishes between critical and non-critical tasks. The rationale for identifying important tasks is that if any one of them is delayed, the entire process would suffer. The essential path is the longest series of actions in a network.

This technique steps as follows -

• A list of all the activities required to accomplish a project is first created
• The calculation of optimistic time required to perform each of them is calculated.
• The dependencies between the activities are then determined.

The difference between PERT and CPM is also based on the time taken to complete a task. We can differentiate between PERT and CPM in that the former emphasizes predominantly time, whereas the latter highlights the time-cost trade-off. The two scheduling algorithms have a common approach to network design and determining the critical path. They are used in mutual inclusiveness with one another to ensure the effective completion of a project.

PERT and CPM difference

The table will help you distinguish between PERT and CPM.

Key differences between PERT and CPM

We have learned about the general similarities and contrasts between the two methods. Now it's time to target the most important points of polarity to understand the PERT CPM difference.

1. The first difference between PERT and CPM is that the PERT technique is a project management technique that involves planning, scheduling, organizing, coordinating, and managing unpredictable and uncertain activities. CPM is a statistical project management approach that involves the planning, scheduling, organizing, coordinating, and controlling of well-defined tasks.
2. The second difference between PERT and CPM that PERT is a time management and planning approach. Unlike CPM, which is a cost and time management strategy.
3. The third difference between CPM is that the PERT technique began as a research and development initiative, whereas CPM began as a construction project.
4. Another difference between PERT and CPM is PERT networks are based on occurrences, whereas CPM is focused on activities and is an activity-oriented technique.
5. A crucial difference between PERT and CPM is that CPM employs a deterministic model. PERT, on the other hand, employs a probabilistic model.
6. The PERT CPM difference is highlighted in another way. In PERT, there are three-time estimates: optimistic time (to), most probable time TM, and pessimistic time (to) (tp). CPM, on the other hand, has only one estimate.
7. The PERT method is better for a high-precision time estimate, whereas CPM is best for a realistic time estimate. So, time estimate elaborates PERT vs CPM difference.
8. PERT technique is for unexpected activities, whereas the CPM technique is for predictable ones.
9. The PERT vs CPM difference is also seen when the nature of the task is non-repetitive, PERT is applied. CPM, on the other hand, is a task that is repetitive in nature.
10. In CPM, there is a distinction between critical and non-critical activities, which does not exist in PERT charts. This is an important difference between PERT and CPM.
11. PERT is appropriate for research and development projects, while CPM is best for non-research tasks such as construction projects.
12. Crashing is a CPM compression technique used to reduce project activity duration while minimizing additional costs. The idea of crashing does not apply to PERT and becomed an important factor in the difference between PERT and CPM.

The components of the CPM and PERT network - Activity and Event

An activity depicts action and the use of resources such as money, time, and materials needed to execute a project. An arrow is used to signify activity.

There are two sorts of activity in the network:

• Preceding Activity: Activities that take place before a specific event.
• Succeeding Activity: Succeeding activities are those that occur following a specific event.

Event - At the start and finish of the activity, there will always be an event. Using Fulkerson's RULE, number the event in–network.

• Dummy activity- A dummy activity is a made-up activity that doesn't take up any time or resources. The dotted line denotes dummy activity. It provides a link between events in order to keep the sequence rational.
• There are no resources available for an event. A node represents an event.
• There are several sorts of events in the network, including head, tail, merging, and burst events.

Critical Path Analysis

1. The longest path in the whole project network is the critical path for any network. When all of the actions have been completed, the project will be finished.
2. All actions on the critical route must begin as soon as feasible if the project is to be finished in the shortest period possible. These are referred to as key activities.
3. The project's duration will be higher if the crucial activity is delayed. Any sort of delay is taken into account here, including severe weather, accidents, and technological issues. The duration of a project has a direct impact on its budget.
4. Non–critical actions are those that are not on the critical path. There might be some slack in these non-critical duties.
5. The slack is the amount of time that an activity may be postponed before it has an impact on the project's overall completion time.
6. To minimise the entire project duration, more resources, such as manpower, materials, and machinery, would be required at an additional expense to speed up the completion of important operations.

How to calculate project completion time

Consider this data given-
1. Construct the PERT network.
2. Find the Earliest Time (TE) and Latest Time (TL) for each activity.
3. Find the critical path.

Consider the activities and construct the activity network diagram as-

Calculation of TE -

TE 1 = 0

TE 2 = TE1 + t1,2 = 0+4 = 4

TE3 = TE1 + t1, 3 = 0+1 = 1

TE4 = max (TE2 +t2, 4 and TE3 +t3,4)

TE4= max (4 + 1 and 1+ 1)

TE4= 5 Days

TE5 = TE3 + te3, 6 = 1+6 = 7

TE6 = TE5 + te6 = 7+4 = 11

TE7 =15

TE8 = 17

TE9 = 10

Calculation of TL -

TL10 = TE10 =22

TL9 = TE10 – t9,10 = 22 – 7 = 15

TL8 = TE10 – 18,10 = 22 – 5 = 17

TL7= TE8 – t7,8 = 17 – 2= 15

TL5 = 7

TL4 = 10

TL3 = 1

TL2 = 9

TL 1= 0

Although PERT and CPM are like two different poles, due to their likeness, they are getting used as a conjunction in various projects. The CPM and PERT difference can be seen in that the former thrives just on time as a variable with the idea that cost will automatically get reduced as time down surges. The latter aims at both minimizing the time as well as additional cost. It will be more difficult to distinguish between PERT and CPM over the course of time.