Written by T. E. Snyder
Image Credit: Pixabay
What Is a PERT Chart
How did project managers ever figure out the critical path before MS Project was created? We used PERT!
Have you seen a PERT chart wondered how it works? This article will attempt to demystify the PERT and assist in developing a clearer appreciation for what this technique offers.
A PERT chart is a project management tool that identifies (3) three main outputs.
- Graphical representation of the “flow” of project tasks, successors, predecessors, timeline
- Identifying the critical path of your project
- techniques to develop sound task estimations
PERT stands for Program Evaluation Review Technique.
Gantt vs PERT Chart
Although PERT and Gantt charts are both visualization tools that project managers use in displaying the tasks which are required for project completion, the PERT provides a visualization of the flow of tasks. The difference between them is that a Gantt chart is basically a bar chart and a PERT chart is a flow chart.
History of the PERT Chart
Program Evaluation and Review Technique charts were developed and introduced in 1950 by the U.S. Navy. They were developed to manage large projects which had complex tasks and a very high inter task dependency. The charts have an initiation node, and the initiation node later branches into many networks of tasks.
PERT charts represent projects which need an assembly line to complete a project and work well with the Waterfall methodology. The PERT chart has many interconnecting or parallel networks of independent tasks.
These charts are designed for small parts of the project. They end at the main point of review. The limitation of a PERT chart is that they can be very confusing and complex; thus, they are used along with Gantt charts which are simpler and more straightforward.
- Gantt charts were developed and introduced in 1917 by Charles Gantt. It deals with the sequence of tasks needed to complete the project; whereas PERT charts were developed and introduced by the U.S. Navy in 1950 to manage large and complex projects.
- Gantt charts focus on the time required to complete a task; whereas a PERT chart focuses on intertask relationships.
- Gantt has linear representations or it is a bar chart; whereas a PERT chart is a flow chart and has parallel networks of individual tasks.
- Gantt charts are straightforward and are not made for projects which need changes; whereas PERT charts are complex and are made for small portions of the project.
How Do PERT Charts Work?
A PERT chart uses circles or rectangles, called nodes, to represent project events or milestones. These nodes are linked by vectors or lines that represent various tasks. Dependent tasks are items that must be performed in a specific manner.
For example, if an arrow is drawn from Task No. 1 to Task No. 2 on a PERT chart, Task No. 1 must be completed before work on Task No. 2 begins.
Items at the same stage of production but on different task lines within a project are referred to as parallel tasks. They’re independent of each other, but they’re planned to occur at the same time. A well-constructed PERT chart looks like this:
PERT Probability Approach to Project Scheduling
Activity completion times are seldom known with certainty. PERT is a technique that treats activity completion times as random variables. Completion time estimates can be estimated using the Three Time Estimate approach. In this approach, three time estimates are required for each activity:
o = an optimistic time to perform the activity
m = the most likely time to perform the activity (mode)
p = a pessimistic time to perform the activity
With the three time estimate approach, the probability distribution for activity completion time can be approximated by a Beta distribution.
Mean and Standard Deviation to develop estimates
The standard estimate of the mean is a weighted average of the three time estimates with weights 1/6, 4/6, and 1/6 respectively on o, m, and p variables. This derivation of a standard deviation is often called the “standard error” of the estimate or “standard error of the mean” when referring to a mean (or average).
Since most of the area lies 3 standard deviations on either side of the mean (6 standard deviations total), then the standard deviation is approximated by Range/6.
It can be represented with this calculation:
Dur = (O + (4 * M) + P) / 6
Example: Task (A) has a Optimistic (o) value of 6.5 hours, a Most Likely (m) value of 7.5 and an Pessimistic value (p) of 9.0 hours.
The mean value would be:
(6.5 + (4 * 7.5) + 9.5) / 6
(6.5 + 30.0 + 9.5) / 6
46 / 6 = approximately 8 hrs (rounded up)
Forward and Backward Pass
Step 1 – Add durations
Add durations to each task based on the Estimate exercise above
Initial Early Start is almost always “0”
ES (Early Start + Duration) = Early Finish (EF)
Proceeding EF is the Early Start for successive tasks
Where tasks converge (F), the ES (Early Start) will be the MAX value of EF (Early Finish) of the proceeding dependant tasks (C and D).
Example 2 of converging tasks. Again, Take the MAX value of Early Finish (EF) in Task B and F to populate the (Early Start) in Task G
Task H (Deliverable) Late Finish equals Early Finish.
Calculate the Late Start by subtracting Duration from Late Finish
Carry Early Finish (H) to the Late Finish on Tasks E and G.
Backward Pass completed
During the Backward Pass, the MIN value from Late Start From Tasks (B, C, D, E) is passed to the Late Finish of Task A.
Time to calculate for Critical Path.
Critical Path is identified when the “slack”is zeroed.
** It should noted that it’s possible to have several critical paths.
The days of creating a PERT may be a thing of the past. Although it’s important to understand how the critical path is identified, most project management practitioners recognize the simplicity of how MS Project will create a PERT output rather quickly.
Personally, I use PERT standard estimation to calculate task durations and use these to populate my MS Project schedules.
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