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July 07, 2023

PERT chart vs. Gantt chart: key differences and similarities

Gantt Project Management Scheduling & Roadmapping Scope
Agnieszka Sienkiewicz

PERT charts and Gantt charts are two valuable project management tools managers often use to plan and keep track of projects. Both share some similarities but differ in a few important ways. How do they compare, and what makes one better than the other in a given situation? So happy you asked! Read on for the answers.

What is PERT?

PERT is an acronym for Project Evaluation and Review Technique (some sources say “program” instead of “project”). In essence, it’s a probabilistic (statistical) method of analyzing the tasks involved in completing a given project. In particular, analyzing and estimating the time needed to complete each task and calculating the minimum time needed to complete the total project.

PERT uses the CPM (Critical Path Method) to determine the longest path through a project. Using this approach, you can figure out the earliest and latest completion dates of individual events and time reserves on individual tasks. By using PERT and CPM together, you can calculate the minimum time you’ll need for project completion.

What is a PERT chart?

In its graphical form (a chart), PERT is visualized as a directional linear network diagram or flow chart. The diagram provides a visual representation of a project’s timeline and breaks a project down into individual tasks. But unlike on the Gantt chart, PERT charts have no fixed timeline with dates on the X-axis. 

PERT charts also take into account relationships between individual tasks (dependencies) and the priority of tasks in relation to each other (which one starts or finishes first). 

There are three typical components of PERT charts: 

  1. Nodes (rectangles or circles representing projects, activities, milestones, or tasks)
  2. Arrows or Vectors (lines connecting the nodes representing tasks or dependencies)
  3. Time estimates (for individual project activities or tasks):
  • Optimistic time — the minimum amount of time required for task completion.
  • Most likely time — the maximum amount of time required for task completion.
  • Pessimistic time — the best estimate of the time it will likely take to accomplish a task.

The nodes and arrows can have different meanings depending on the notation you use, and your choice will determine how you’ll draw and read the PERT chart. There are two notations you can apply: Activity on Arrows (AoA) and Activity on Nodes (AoN). 

Activity on Arrows (AoA) notation on a PERT chart 

As the name suggests, this notation means that arrows connecting the nodes represent the tasks (activities). The nodes, on the other hand, represent project events or milestones. The back of the arrow indicates the start of the task and the front points to the end. Arrows can go in only one direction. For that reason, the AoA diagrams support only the Finish to Start relationship (=Finish to Start dependency).

On AoA diagrams, the first node represents the start of the activity (=start event). The second node represents the finish of the activity (=finish event). The name of the task sits above the arrow and its duration goes below.

An illustration showing how Activity on Arrow works.

Under both nodes, you can see EET and LET times, standing for Earliest Event Time and Latest Event Time respectively. The EET of the first node is the earliest start of the activity and the LET is the latest start. Likewise, the EET and LET of the second node are the earliest and latest completion dates of the activity. The sum of all activity times must equal the “total project duration” (the time it will take to complete a project).

How do you find the values for EET and LET? They come from your critical path analysis.

Activity on Node (AoN) notation on a PERT chart

Using the AoN notation, nodes represent project activities that are connected from beginning to end with arrows. Those arrows depict a logical progression of dependencies between the scheduled activities. 

Because the activities are placed on the nodes, the arrows can go in any direction. As a result, they can represent several types of dependencies (Start to Finish; Start to Start; Finish to Start; Finish to Finish). (This is different from the AoA diagram because arrows using that notation can go in only one direction). For that reason, AoN is more popular.

Each AoN diagram node can have a letter (or number) that correlates to activity on the project schedule. Alternatively, you can insert the name of the task directly into the node.

An illustration showing how Activity on Node works.

When to use PERT charts

PERT considers three different time estimates to get the expected activity time according to the following formula: 

Mean estimate = [Optimistic Time + (4 x Most Likely Time) + Pessimistic Time]/6

PERT formula for calculating mean time.

Such a statistical approach makes PERT charts useful when you want to schedule your project, even if you don’t have precise or definite values for the duration of all project activities. This makes PERT a great choice for planning and scheduling R&D projects when determining precise times for individual activities is difficult (or even impossible).

You’ll also find PERT charts useful in the project planning phase when you need to map out the full project scope. PERT gives you a high-level overview of all the tasks you’ve included in the project, along with their dependencies and estimated completion dates. And because it’s a flow chart, where each task points directly to the next sequential task (or tasks), a PERT chart provides an excellent visual representation of the relationship between each project activity. 

PERT chart: advantages

  • A clean and minimalist overview of a project helps you identify the important success factors (e.g., critical path, key tasks).
  • A statistical calculation method makes it possible for you to plan projects with uncertain activity durations.
  • Suitable for optimizing project completion time (but not project cost).
  • Because it’s a network diagram (flow chart), a PERT chart provides an excellent visual representation of the relationship between each project activity.
  • Identifies the critical path very well, which helps you hit deadlines and deliver the project to clients on time.
  • Allows for individual activity analyses.
  • Helpful in conducting what-if analysis because it emphasizes critical and subcritical paths.

PERT chart: disadvantages

  • Linear PERT charts can get complicated or confusing in complex projects with hundreds of tasks, milestones, and other deliverables.
  • Very labor-intensive to create and challenging to update while the project is underway.
  • PERT places a very heavy emphasis on time, which makes it an inflexible tool for project management.
  • Not the best tool for tracking resources you need to share between three (or more) projects.
  • Not the best choice for long-term projects when you need to add new tasks as the project progresses. (Might also involve more resources in the process, making project tracking challenging.

What is a Gantt chart?

Gantt charts, like PERT charts, break projects down into smaller activities and highlight scheduling constraints with dependencies. Each activity is represented with a bar. The length of the bar depends on the duration of the activity.

Unlike a linear PERT chart, a Gantt chart is a chronological two-dimensional structure that progresses along the timeline (X-axis). Simultaneously, it cascades downward along the respective date columns (Y-axis). Each task and project event has a corresponding element on a chart.

On a Gantt chart, the WBS (work breakdown structure) work items cascade along the Y-axis. The project timeline grows along the X-axis.

 

Typically, a Gantt chart has five main components:

  1. Timeline (progressing along the X-axis).
  2. Taskbars (horizontal bars visualizing tasks and their duration).
  3. Dependencies (arrows connecting taskbars).
  4. Baselines (horizontal lines under taskbars serving as scheduling reference points).
  5. Milestones (important events, visually distinguished from project tasks).

Here, the length of a bar for each task (a user story or other issue type) matches its duration exactly. So the longer the time estimate for a certain task, the longer the taskbar gets. In this regard, Gantt chart bars provide more visual information about a task than nodes and arrows on a PERT chart.

Gantt chart (right section) along with a work breakdown structure (left section). The lines below the taskbars represent baselines. The arrows connecting the bars visualize dependencies. (Source: BigPicture)

 

As with PERT charts, you can add a critical path to a Gantt chart. The taskbars (like PERT nodes) also enable you to place the names and time estimates for individual tasks directly on them. You can also assign individual resources to each activity and set execution priority

Task (cross-project) dependencies and lag time

Task dependencies on a Gantt chart can go in either direction. This means you can build Start to Finish; Start to Start; Finish to Start; and Finish to Finish relationships between project items. You can also create multiple but different dependencies from one task to several others, even if some of the tasks are outside the scope of your project.

Strong dependencies between tasks. All of those dependencies have a scheduling impact on the dependent (successor) tasks.

 

But a large number of dependencies clutters things up, making project tracking on traditional Gantt charts very difficult. For that reason, the BigPicture app offers you a variety of smart features to help you deal with “dependency spaghetti” in your project. You’ll also be able to add a lag time and ASAP mode to task dependencies for more flexibility. And whenever you make a change in the schedule of the predecessor task, the app will auto-reschedule the successor task. 

When to use a Gantt chart

Gantt charts work well in the project execution phase because you can adjust them if the scope changes. (And when does the scope not change during a project?)

Each task has precise start/end dates that you can read from the chart timeline (or WBS column). And if there’s a delay, the critical path along with baselines helps you assess the impact. 

Gantt charts are also great for planning resources — they can track each person’s task allocation and schedule throughout the entire project lifecycle. You’ll find them particularly helpful when you need to share your resources across overlapping projects or tasks. Or when there are multiple complex projects requiring the same skills/resource role. 

(That kind of information would be difficult to get from traditional Gantt charts. But using some modern Gantt chart software, you can easily do it.)

Resource Panel listing project resources and their current capacity status in BigPicture.

 

And because Gantt charts are visual project management tools, you can use them for project reporting as well. Even in their traditional form, they can show stakeholders your progress towards completion because of their chronological structure. If you couple them with PPM software, you can also export your Gantt chart view and generate dedicated reports.

Gantt chart: advantages

  • Provides an overall perspective of a project and its timeline for every stakeholder using visual elements, like bars, lines, arrows, and special shapes or symbols (e.g., diamonds for milestones).
  • Aids in effective resource management, especially helpful when you need to share resources with other projects.
  • Enables project leaders and members to closely track tasks, milestones, and overall workflow.
  • Clearly illustrates how tasks in a project might overlap and how the beginning of one task can be dependent on the completion of another
  • Suitable for tracking and balancing multiple projects by highlighting delays, task overlaps, resource overloads, and activities that burn too much time.
  • Lays out all dependencies — as well as tasks and other project deliverables — on an easy-to-follow Cartesian plane. This clear visualization enables you to get buy-in on realistic deadlines for each task (and for the entire project)
  • Helps to follow a project by day, week, month, quarter, and year.

Gantt chart: disadvantages

  • When working on multiple projects at once, project execution, monitoring, and control can get messy.
  • When a project requires setting up and working with too many tasks and subtasks, managing the project becomes challenging.
  • Taskbar length shows only the duration of a task, not how much work it requires for completion.
  • Depending on the project size, the entire project might not be visible in a single view.
  • Without access to modern software, it is impossible to update, assign, track, or schedule a Gantt chart with project tasks and activities.

PERT chart vs. Gantt chart: Visual tools for project management

The key ingredient to successful project management is healthy and effective communication. Both PERT and Gantt charts help you break projects down into smaller components, communicate complex plans, illustrate task connections, and keep everyone on the same page. They can also help increase productivity and make the workflow more transparent for everyone.

The main differences between the two are how they visualize the project structure (linear vs. two-dimensional), how they approach task estimations (estimated task durations vs. accurate due dates), and which stages of the project they fit best (planning vs. execution). 

But you can plan AND execute projects effectively using a Gantt chart alone. Powerful PPM software like BigPicture gives you the ability to break down, estimate, and assign project items. Then you can track them with the help of critical path, baselines, data aggregations, percentage progress bars, and more.

Not a BigPicture user yet? Start your free 30-day trial today. Or visit our demo page to play with the app straight in your browser — no registration or installation needed.