How to Determine a Critical Path

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The ability to determine the critical path of a project is an essential skill for any project manager.
The ability to determine the critical path of a project is an essential skill for any project manager. (Image: process flow image by Christopher Hall from Fotolia.com)

The Critical Path Method was developed in 1957 by J. E. Kelly of Remington Rand and M. R. Walker of DuPont to facilitate the construction and maintenance of chemical plants owned by DuPont. Determining a critical path is a methodology that identifies which activities must be finished on time to avoid a delay in the completion time of the whole project. If one of those activities is late by one day, the project completion date will be extended by one day.

Things You'll Need

  • List of activities
  • Map of dependencies

Develop a list of tasks required to complete the project. The criteria for entering a task on the list is that its completion cannot be delayed without delaying the completion of the entire project.

Identify which activities are dependent upon other activities before they can commence. These activities are called “dependencies” and the activities they depend upon may be parallel tasks that may, or may not be included in the critical path.

Calculate the ES (earliest start) and EF (earliest finish) times for each activity in the project by making a forward pass through a preliminary draft of the activity diagram. The ES is the earliest possible start time assuming that all necessary precedent activities have been completed. The formula used for this calculation is: EF = ES + t The "t" represents the time required to complete that activity, in hours, days, or weeks.

Calculate the LS (latest start) and LF (latest finish) times for each activity in the project by making a backward pass through the preliminary draft of the activity diagram. LS is the latest possible start-time for an activity, assuming that all necessary precedent activities have been completed. The formula used for this calculation is: LS = LF – t.

Chart the activities, their dependencies and time bars into a project scenario. Do it manually with a horizontal spreadsheet or use a project-planning tool for complex projects. Project managers use dedicated critical path software to set up activities and dependencies and calculate the critical path. If a delay is entered into the software, it recalculates the completion date of the project. If the completion date is affected, the manager utilizes the software to revise the order of activities or suggest additional tasks to help put the project back on track.

Tips & Warnings

  • As soon as it is completed, the project’s critical path becomes a moving target that must be continually monitored and adapted according to how events unfold.

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