What Is PERT in Project Management? PERT Analysis Model Guide

Wednesday, August 3, 2022
Michael Scheiner
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Welcome to this PERT analysis article. Here, we will go over the major question: what is PERT in project management? This text will go over definitions, key terminology, the steps and formulas of PERT, as well as pros and cons. 

Are you ready to get your PERT on? Buckle up.

 

What is PERT in project management?

So, the big question, what does PERT stand for in project management? The PERT full form stands for program evaluation and review technique (PERT). That is, it is a method for planning, evaluating and reviewing a project or program based on time estimates and task dependencies.  

So, what is the program evaluation and review technique? Our definition

PERT techniques are especially effective at time management. A PERT technique offers tools for planning for the best-case scenario, the worst-case scenario, and everything in between. 

Our PERT project management definition centers on the fact that it’s one of many project management techniques for understanding the sequence and dependency relationships between tasks as part of a larger complex project. 

PERT is generally implemented by various types of PMO (project management office), whether that be supporting, controlling, or directive.

Where did PERT networks originate?

You know PERT is a fantastic project management tool for complex projects because of its dark origin story. It was developed by the special projects office for the U.S. Navy’s Polaris project in the 1950s for managing the construction of nuclear submarines. Whoa!

Ever since then, PERT has been popular for complex project planning.

 

PERT method project management terminology

It’s always wise to brush up on the lingo of anything you are learning about. Below you’ll find the key terms and definitions related to the PERT method in project management.

And if you’d like to take a step back and ask: what does project management mean in general? We have an article about that. Now onwards.

Here’s the key terms at a glance:

  • Network diagram

  • Nodes

  • Milestones

  • Arrows

  • Dependencies

  • Predecessor

  • Successor

  • Lead time

  • Lag time

  • Constraints

  • Slack

  • Critical path

  • Critical path activity

  • Dummy activity

  • Fast tracking

  • Crashing

  • Relaxation

  • Work breakdown structure (WBS)

Network diagram

A network diagram is the kind of chart that PERT uses. It is a wireframe diagram which connects events to one another with lines.

Nodes

When you look at any wireframe chart or network diagram such as the PERT chart, the nodes are the icons, often squares or circles, which represent events and are connected by lines.

Milestones

In the PERT method, a milestone is the end result of a task, or the competition of work activity. In these charts, PERT events and milestones are the nodes.

Arrows

Arrows represent tasks, and often display the estimated time to complete them. As we’ve said regarding network diagrams with nodes and lines, the PERT chart often uses arrows for lines to show the direction that work must take as well as any task dependencies.

Dependencies

A task dependency is a crucial concept in all project planning. It refers to the relationship between tasks and whether any task must be started or completed before another task can be completed or started.

Predecessor

A predecessor, or preceding task, is a task in a dependency with a later task, meaning this predecessor must either be completed or started before the next one begins or ends.

Successor

Like the predecessor, the successor, or succeeding task, is the later task in a task-dependency relationship along a project timeline.

Lead time

A lead time is a measure of how long a successor task can be moved up in the project scheduling in relation to its predecessor. 

Lag time

Lag time is how much time the project team members can delay a successor task without affecting the predecessor task.

Constraints

Constraints are like dependencies but add more detail to individual tasks. Constraints are also related to start and end dates, for example: ASAP, ALAP (“as late as possible''), MSO and MFO (“must start/finish on”).

Slack

Slack time is all about the amount of time that a task can be put off or delayed without affecting the completion date of the project, or negatively, how much time must be saved or won back. Slack is also known as float time.

Critical path

The critical path is the longest amount of time projected to complete all the tasks in the entire project, taking into account all the most important tasks and their dependencies. It is, in other words, the longest path.

Critical path activity

Critical path activity refers to the most necessary steps in the project that are fundamental to its completion, and taken together make up the entire project duration of the critical path.

Dummy activity

A dummy task or dummy activity is not a real task but a tool of the diagram to show an indirect relationship between other tasks.

Fast tracking

Fast tracking is what you can do to speed up the project by green lighting several tasks from different stages to be done more simultaneously.

Crashing

Crashing is also used to speed up the project but instead of restructuring task scheduling, it is done by shifting resources to certain tasks.

Relaxation

Relaxing your activities, or relaxation, is the opposite of crashing. Relaxation is often used to lower costs when scheduling is not so important or when you have lots of slack time.

Work breakdown structure

WBS, or work breakdown structure, is a simple top-down tree chart which breaks the project down into task groups and subtasks, and is useful to have before planning a PERT chart.

 

When to use the PERT model

PERT models are useful in many cases. However, perhaps it is best to first describe the scenarios where PERT is more appropriate than other similar methods and tools.

PERT vs the critical path method

The CPM, or critical path method, is a project management methodology which emphasizes the use of a critical path chart. In this case, a PERT chart is more useful than a CPM chart because of its use of 3 distinct time estimates which produce an expected time, whereas CPM focuses on just a single longest time estimate.

CPM charts also differ from PERT because the nodes in the critical path method represent tasks and activities, whereas in PERT the nodes represent results and milestones. That’s PERT vs CPM in a nutshell.

 All in all, you’d use PERT over CPM to allow for more flexibility in planning. 

PERT vs Gantt charts

Gantt charts are project management tools used in project planning to lay out tasks, duration of tasks, and to see clearly which tasks overlap with one another. Gantt’s are less useful than PERT’s to see task dependencies. Gantt charts use bar graphs and calendar dates, while PERT charts are flowcharts which emphasize task durations and project completion times.

You’d use PERT charts over Gantt templates at the outset of complex project planning, whereas Gantt charts are something a project manager can tinker with throughout a project lifecycle. 

 

The 4 PERT time estimates

When discussing time in PERT, there are four main types of time estimates.

1. Optimistic time estimate

The PERT optimistic time estimate is an estimate of the minimum amount of time an activity will require. This best estimate assumes there are no problems or setbacks and everything goes as planned.

2. Pessimistic time estimate

The time an activity will take assuming very unfavorable conditions is the pessimistic time estimate. Here you assume every delay or problem that your foresaw will occur, and therefore assumes the maximum amount of time.

3. Most likely time estimate

This is the reasonable estimate for the duration of a task. It assumes there might be some minor problems or delays that have a high degree of occurrence, and factors in those expected delays.

4. Expected time estimate

This last PERT time is the estimated amount of time that you get as a result of applying the PERT formula. This expected time is often what you will see on the arrows which represent the project tasks.

 

PERT analysis formula

In PERT, the activity duration time is equal to the results of this formula using the first three time estimates:

E = (O + (4*M) + P) / 6

Where:

  • E: Estimated time

  • O: Optimistic time

  • M: Most likely time

  • P: Pessimistic time

This means that with the PERT algorithm, your estimated time is a weighted average of optimistic, pessimistic and most likely estimates, with the most likely time given 4 times the weight in the average. Add up all the task expected times and you get the expected time for the whole project.

There is another useful PERT algorithm to calculate the standard deviations, which is:

(P-O) / 6

Meaning when you subtract the optimistic estimate from the pessimistic estimate and divide by 6, the resulting number tells you how much you may deviate from the estimate. The larger the number, the wider the range and the less exact your estimate will be.

 

PERT analysis example: The 5 PERT steps

There are 5 steps when implementing PERT, which we’ll flesh out with a program evaluation and review technique example.

1. Identify project milestones 

The first step in the PERT method is to plan the project based on milestones, with the final milestone being the completion of a project.

  • Example: Setting up an online shop, the milestones can be: website design, finish uploading a product catalog, finish creating a marketing campaign, website launch.

2. Plan task sequence and dependencies

The second PERT step is to figure out all the necessary tasks, possibly using a WBS. Place the tasks in sequential order, and make it clear if you have any task dependencies.

  • Example: In website design, you cannot finalize the design before the graphic art department has completed creating its assets.

3. Connect the tasks on a PERT diagram

Now that you have your milestones and tasks, it’s time to create the PERT diagram network wireframe. Milestones are nodes, tasks and activities are the arrows connecting them.

  • Example: Connect the tasks of planning the marketing campaign to creating the text and art for the campaign, which lead to the milestone of launching the campaign.

4. Do task time estimates

For each task and work activity, do the formula using the optimistic time, the pessimistic time and the most likely time to figure out the expected time and add those figures to the arrows which represent tasks.

  • Example: Collecting text and images of each product for your online catalog will optimistically take 30 hours, pessimistically take 60 hours, and the most likely time is 40 hours. Your expected time is then 43.33 hours.

5. Identify the critical path in PERT

Finally, you need to identify the critical path of your project, which is the timeframe encompassing all the most important tasks from the first until the last one, which informs you of the total completion time.

 

PERT project management advantages and disadvantages

Here are some pros and cons to the PERT method:

Benefits of PERT

The first pro of PERT is that it makes complex projects more manageable by ordering tasks and identifying the critical path.

PERT is also great for being ready for a variety of changes to the duration a task will take, either by being completed fast or by being hindered by obstacles.

Finally, PERT is great for clearly communicating project timeframe expectations to stakeholders. 

Drawbacks of PERT

One main con regarding PERT is that some managers feel it can be rigid and inflexible once the diagram is done and the project is put into motion.

Another disadvantage to PERT is that it is very labor intensive, as it does require a lot of work at the project planning stage.

Finally, some people think PERT is less accurate with task planning because of its range of time estimates.

 

Key takeaways

For those super interested in PERT, there may be some forms of certification like a PMP title, or project management professional, but you’ll have to do your own research there. 

Ultimately, PERT is a great tool for the seasoned project manager but also useful for anyone planning a complex project while availing themselves of the best project management software out there.

 

FAQs

The most realistic estimate of the time required to complete an activity is referred to as what?

In PERT, the optimistic time estimate is the estimate for how long a task will take if everything goes according to plan and nothing goes wrong. The optimistic time is the best estimate, and is used in a formula with the pessimistic time and most likely time.

What is the shortest duration required to complete an activity?

The shortest duration of time required to complete a task or work activity obviously depends on the task. In the PERT system, you are supposed to figure out the best possible time and use it in the formula as the optimistic time in contrast to the pessimistic estimate.

What project management methodology works best for projects with probabilistic task times?

Of all the many project management methodologies, the PERT method is very useful for managing projects with probabilistic time frames. PERT helps estimate expected durations with a formula that uses optimistic times, pessimistic times and most likely time durations to help figure out your expected durations. 

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