PERT vs CPM: What's the Difference Between PERT and CPM Techniques?

Friday, August 5, 2022
Michael Scheiner
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Every project starts with a plan. And planning revolves around three main things: time, money and scope. Using a project management review technique like PERT or CPM can be very useful in planning, especially when it comes to time management. 

But what’s the difference between PERT vs CPM? 

Which one is better for what purposes? This article answers that question and so many more in the widely discussed PERT CPM technique debate.

 

PERT and CPM full form meanings

If you want to know the full form of PERT and CPM acronyms, here they are below in all their glory.

PERT stands for program evaluation and review technique.

CPM stands for critical path method. 

 

Now, let’s define PERT and CPM in project management

OK, the acronyms are out of the way. Just like with appetizers, it’s time to make room for the main course: definitions. Here are the PERT CPM definitions and explanations.

PERT

PERT is a project management and review technique. It is a statistical technique used for getting highly accurate time estimates for complex projects. The PERT technique makes strong use of an algorithm that calculates estimated times for unpredictable and uncertain activities. 

PERT focuses on events and milestones, and the PERT chart emphasizes this by making those the nodes in the wireframe. PERT is very popular in research and development projects.

CPM

The critical path method is also a statistical project planning technique. With CPM, the main focus is on determining the longest path along all the critical activities in a project. The idea is to arrive at a highly reasonable estimate for how long the whole project will take to complete. 

CPM emphasizes the tasks and work activities more than the events and milestones, and for this reason, a CPM network diagram places tasks in the nodes. CPM is also useful for separating critical from non-critical activities. The civil engineering world is where CPM is most useful.

 

What is the difference between PERT and CPM?

The main difference between PERT and CPM is that PERT focuses on time planning and time management, with a goal of delivering the project as quickly as possible. CPM focuses on time and budgeting, which is useful for bringing in the project on budget as well as on time. 

By now, you should have a better idea about how to answer the question: what is PERT used for? You should also have a good idea of how PERT is different from critical path method in terms of emphasis. 

But fret not if you want to know more about PERT CPM techniques and CPM PERT charts, because now we are truly going deeply educational on the topic.

 

Chart to compare PERT and CPM techniques

Comparison

PERT

CPM

Project management objective

Time, scheduling and deadlines

Time-cost tradeoffs

Project orientation 

Event oriented

Activity oriented

PERT/CPM Chart

Milestones as nodes, lines as tasks

Tasks as nodes, lines as sequences and dependencies

Critical activities

No emphasis

Distinguishes critical from non-critical activities

Project model

Probabilistic model

Deterministic model

Time estimates

Three time estimates

One time estimate

Predictability

Best for unpredictable activities

Better for predictable activities

Project repetition

Better for non-repetitive projects

Best for repetitive-projects

Crashing concept

No crashing

Crashing 

Main uses

Research and development projects

Construction projects

 

PERT vs CPM comparison points

Wasn’t that a great chart? You know what else has great charts? Project management and review methods like PERT and CPM. It is time for the most spectacular CPM vs PERT studies ever undertaken. Here we go.

Project management objective

It’s pretty safe to say that when comparing project management tools, the main difference between CPM and PERT is based on the tool’s focus and objective. When it comes to PERT, the focus is on project planning, timeframes, scheduling and getting the whole project completed on deadline. 

CPM, on the other hand, focuses on the time-cost tradeoffs. In other words, PERT is great for optimizing the amount of time spent on a project, and CPM for balancing the cost of the project within reasonable time estimates.  

Project orientation 

Another big difference between CPM and PERT is the technique of the PM tool. CPM is what you call an activity-oriented technique, which means that what’s most important when planning the project are the tasks, the amount of time spent on tasks, and the resources needed to complete them. 

PERT is an event-oriented technique. This means that what counts most in project planning are events like deliverables and meeting milestones. Events in PERT are basically the result of the project activities in CPM. 

PERT/CPM Chart

Both PERT and CPM use wireframe network diagrams for creating PERT charts and CPM charts. At a glance they do look similar. 

The main difference between PERT network diagram and CPM charts are as follows: The PERT chart places milestones as nodes, which is due to it being event oriented. The lines connecting the milestones are obviously the tasks, which will also display the estimated time of the activity duration.

The CPM chart puts tasks as nodes which include activity duration estimates, using lines to emphasize the sequence and dependencies between the tasks.

Critical activities

As is clear in the name, the critical path method is used for determining the critical path, which is the longest path and duration from start to the project completion time of all the crucial tasks or critical sequence of activities. 

CPM charts also show non-critical activities, which may float in the schedule. PERT, on the other hand, is more about figuring out the optimized timeframes and schedules of all tasks and activities, though you can customize a PERT chart template to highlight critical tasks as well.

Project model

The PERT method is a probabilistic model for approaching project planning. It isn’t aimed at directly instructing project teams on how long they must spend on each task or for the entire project duration. 

Instead, it is an attempt to best estimate how long things will take to help project managers optimize their planning. CPM is not like this. CPM is a deterministic model. It is aimed at giving project teams clear instructions on how long each activity should take.

Time estimates

The CPM relies on a one time estimate. This time estimate is supposed to be a reasonable time estimate, and often includes some factors regarding common delays or problems. PERT, on the other hand, is a much more high precision time estimate tool. 

This is because it uses an algorithm with three time estimates. These are the pessimistic time, the optimistic time that’s the absolute minimum time, and a most likely time, which is sort of optimistic but factors in common delays or problems.

Predictability

Following from the above two differences between PERT and CPM, which are the project models and time estimates, it should not be surprising that PERT is a method which is great at managing and planning for unpredictable activities and events because of its algorithm. 

CPM does not handle unpredictable activities as well as PERT, but it is solid when the project has a more predictable nature.  

Project repetition

This PERT CPM difference also follows from the above differences. CPM, because of its deterministic model, is really useful for projects of a repetitive nature. CPM deals less with uncertainty so it is simple to re-apply its charts to repeated projects. 

PERT is far from ideal if you are mostly doing projects of a repetitive nature, but it is highly effective for first time projects with many uncertain activities. 

Crashing concept

Crashing is a strategy used by project managers and teams when time is running short and deadlines are approaching, especially if you are behind schedule or have experienced delays. 

Crashing is when managers decide to direct additional resources to a task to get it done faster, like more labor-power. You can do crashing in the CPM. However, crashing doesn’t really apply in PERT.

Main uses

CPM was developed in 1957. It grew out of work at DuPont and has always been closely tied to the construction industry especially in huge civil construction projects. PERT was developed not long after in 1958 and has its origins in the US Navy. 

Nowadays it is more used by research projects and other development projects. It can be used for non-research projects as well, but CPM is better for those.

 

PERT and CPM examples

Here is an example of PERT and one of CPM, both using motorcycles, vroom vroom!

Example of PERT

You want to give your buddy a motorcycle for his birthday because he is such a cool bro. You buy a used one really cheap because it’s not working. 

The plan? To fix and clean up the bike in time for his birthday. The more time you manage, the more frills and other souped-up accessories you will put on the bike, like tassel handlebars or studded mud flaps.

 Your PERT tasks are

  • Figure out what’s wrong with the bike. 

    • Optimistic time: 0.5 hours. 

    • Pessimistic time: 8 hours. 

    • Most likely time: 1 hour. 

    • Applying (O+P+4M)/6, you get 1.6 hours.

  • Remove the busted part. 

    • Optimistic time: 0.25 hours. 

    • Pessimistic time: 1 hour. 

    • Most likely time: 0.5 hours. 

    • Applying (O+P+4M)/6, you get 0.3 hours. 

  • Buy a new part. 

    • Optimistic time: 0.5 hours. 

    • Pessimistic time: 4 hours. 

    • Most likely time: 1 hour. 

    • Applying (O+P+4M)/6, you get 0.9 hours.

  • Install the new part. 

    • Optimistic time: 1 hour. 

    • Pessimistic time: 16 hours. 

    • Most likely time: 3 hours. 

    • Applying (O+P+4M)/6, you get 3.3 hours.

  • Clean the bike. 

    • Optimistic time: 2 hours. 

    • Pessimistic time: 4 hours. 

    • Most likely time: 3 hours. 

    • Applying (O+P+4M)/6, you get 1.5 hours.

  • *Add frills with extra time

By making best and worst time estimates and using them with the most likely time to arrive at the expected time, you should have a good idea of how much time you’ll have before his birthday to soup up the motorcycle with extra frills. According to these calculations, the whole project is estimated at about 7 and a half hours.

Since you don’t know what the problem is, and how hard it will be to find the part or fix the bike, this PERT method is a great choice because it lets you factor in those uncertainties.

Example of CPM

In this case, a company wants to bring a brand new motorcycle to the market. This is more of an industrial job, and after the successful completion, many aspects of this project will be repetitive since the idea is to build many new bikes and ship them to stores everywhere. 

The plan? To determine the key tasks and the critical path, this way you know with a high degree of certainty how much time you have for the project. 

Your critical tasks are:

  • Plan, design and engineer the motorcycle mechanically and electrically

  • Set up factory space

  • Acquire all the parts

  • Manufacture the motorcycles

  • Ship the bikes to stores across the country for launch date

Now that you know your critical path and which tasks have sequential dependencies, you can include non-critical activities and place them in the schedule, knowing they have some slack to change if needed.

Your non-critical tasks are:

  • Design the aesthetic style of the motorcycle

  • Have your branding and marketing team come up with a cool name

  • Make a marketing campaign in preparation for launch day when they hit the stores

Obviously these non-critical tasks must be done before the end of the project, and some of them even sooner in the process. You need to factor in some constraints and dependencies, like the latest end date for designing the bike’s look must be before you complete the manufacturing process, though there is nothing stopping your design team from beginning this task as early as possible. 

CPM’s distinction between critical tasks and non-critical activities is very effective at prioritizing and keeping projects on budget despite time constraints.

 

Is PERT or CPM better?

In short, PERT is more useful in most people’s everyday projects so maybe it is better. But in the end that’s subjective. 

There are pros and cons to both CPM and PERT. Many managers will have a good idea of both systems, while some managers may even have taken part in some of the best project management certification programs where they became total CPM PERT Pros.

Both PERT and CPM can be a part of a larger suite of productivity SaaS. The best project management software will often include templates for everything from Gantt charts, network diagrams and Kanban boards.  

And that about does it for this CPM vs PERT piece. You’ve been a great audience.

 
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