Organization is a key component of project management. When creating a project schedule, managers will find both Program Evaluation and Review Technique (PERT) and Gantt charts to be essential tools for successfully completing the project at hand. Both types of charts provide tools for managers to analyze projects through visualization, helping divide tasks into manageable parts.
Visualization
One of the key differences between a PERT chart and a Gantt chart is the way the information is presented. Gantt charts present information in the format of a bar chart. This presentation helps show the percentage of work completed for each task. PERT, on the other hand, displays information as a network model. This means that a PERT chart presents an initial node from which tasks branch out. This helps project managers visualize the sequence of tasks, as you cannot start on the next activity until the one preceding it is completed.
Work Breakdown Structure
One of the key responsibilities of a project manager is to break down the workload into tasks to guarantee that the project will be finished by the deadline. Both PERT and Gantt charts will display the tasks to be completed, but the charts emphasize different pieces. In Gantt charts, the focus is on the percentage completion of each task, without demonstrating the link that two tasks may have to each other. While PERT typically does not show the percentage completed, because it employs a network model, it is easy to see which tasks depend on each other.

Simplification

Gantt charts are ideal for straightforward projects with few interlinking tasks. Gantt charts present project tasks and time allocation as the only two pieces of data. While this is a limitation where there are interconnecting tasks that depend on each other, for more basic projects it is easier to interpret the data in a Gantt chart. PERT charts, while also including project tasks and time allocation, display dependency. Gantt charts are easier to change as a task moves along and it comes closer to completion.

Accuracy

Gantt charts are simpler to read, but PERT charts extend an element of detail to the project scheduling through both the network model's ability to display dependency and PERT's unique ability to anticipate actual time that a task will take to completion. Tasks in PERT charts will have three representative time structures: optimistic, most likely, and pessimistic. By averaging these times, a manager can predict how long a task will actually take to complete more realistically than the single time that Gantt charts provide.

Expected Completion Time

A strength of PERT/CPM charts is their ability to calculate exactly how long a project will take. PERT/CPM provides managers with a range of time in which the project should be completed, based on the total of all minimum and maximum time limits for all activities. This gives companies a number of advantages, such as the ability to tell customers exactly when their orders will be filled, or to know exactly when to order new supplies.
The expected completion time of the project is based on ideal situations, however, and does not take into account the possibility of unforeseen events. The expected completion time of all subsequent activities and the project as a whole can become skewed when things go wrong, which can cause problems if the company has made plans that rely on the timely completion of the project.
Another weakness of PERT/CPM is that the technique relies on past data and experience to formulate completion time predictions. New companies may not have any past experience to lean on, putting them at a disadvantage.

Efficiency

Businesses can share PERT/CPM charts among all key employees, letting employees at each station know exactly when they will be required to begin work processes, where the required inputs will come from, where the outputs must go, and when their task must be completed. This can help dispersed employees to operate efficiently by having a common understanding of the expected work flow.
When things go wrong, however, the very thing that encouraged efficiency might suddenly cause confusion. When a project is held up due to an unforeseen circumstance, workers at all subsequent stations must delay their own progress while explaining to subsequent stations' employees why outputs are not flowing.

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References and additional resources:
http://www.ask.com/question/pert-chart-vs-gantt-chart
http://smallbusiness.chron.com/difference-between-gantt-charts-pert-charts-43848.html