Scheduling: Critical path, PERT and Gantt

Developed by Frank Almirudis, currently under construction

Title to be changed to: Choosing critical path, PERT or Gantt as your project scheduling method

Abstract

Under the aspect of Complexity in project management, Scheduling is the development and control of a plan which details when and how the products or services defined in a project scope will be delivered.

The resulting plan is known as a project schedule, which can also be used as a tool for management of stakeholders' expectations, means of communication and/or evaluating performance. Project data like planned dates, activities, resources, constraints, dependencies and activity sequence and duration are all inputs to the project schedule, and should be constantly monitored and updated by the project management team. To properly create and use the project schedule with the aforementioned inputs, the team must select the scheduling method that best suits the needs of their project.

Three scheduling methods are covered in this article:

• Critical path method (CPM)
• Program evaluation and review technique (PERT)
• Gantt

The focus of this article is to help the reader:

• Know the definition, theory and purpose behind each scheduling method.
• Understand under what circumstances each method is best applied.
• Decide which scheduling method fits their case best.
• Understand the limitations of their selected method.
• Reference the reader to different sources on how to apply their selected method.

Scheduling methods

Critical Path Method (CPM) is a technique created to identify the key activities that are critical for the completion of a project, bearing in mind that if any of these were to be delayed, then the whole project delivery would be delayed. The end game of this method is to find and optimal balance between the length of the project and it's associated costs, as well as identifying the costs of the activities are minor according to the optimal time. In order to create a critical path diagram, one must at least consider all the activities that are requied to deliver the project, their duration and their dependencies. As these components are similar to the ones used in PERT, the two methods are often used together and their combination is called PERT/CPM.

Program evaluation and review technique (PERT) is a scheduling method aimed to create a project's timeline in a graphical manner. This timeline helps the user monitor and control the project's activities while ensuring that the interdependencies between said activities are kept in check and complied to in order to deliver the project. While activities are the central component of PERT, they are not scheduled, this is because the interdependencies dictate when an activity is able to be completed, as all other activities preceding it must be fulfilled before the next activity may be done. PERT diagrams are integrated by 4 key elements: Activities, preset date, calendar and contrains.

• Activities are sets of errands that are to be done to complete the project, these should have a completion time and a description.
• Preset date are set to the activities that may critically affect the delivery, and these dates are aimed to be respected.
• Calendar of working and non working days to accurately respresent the time when this project is to be completed.
• Constrains to set the interdependencies of the activities, these contratins can be: finish-to-start, finish-to-finish, start-to-start or finish-to-finish.

The timeline using PERT is usually represented by using two methods: Activity On Arrow (AOA) in which each activity is represented by arrows, while nodes are starting and finishing events of the activities. The other method is Activity On Node (AON) where activities are repesented as nodes while the arrows are the interdependencies of the activities. Note that an activity may not be started until all preceding connected activites are fulfilled.

Best suitable method for particular cases

Here I will write about indicators that the reader should pay attention to in their particular project, and which of the methods they should choose according to their indicators