How to Calculate a Critical Path Backward Pass


To complete projects on time, the individual steps involved in a project must be completed on time to keep things moving. In a project, some tasks are more important than others, some take more time to complete, and some cannot begin until others are completed. A backward pass is one of the steps project managers take to determine the critical path in a project, which consists of only the minimum tasks necessary to get a project moving.

Starting with the last task in the project, calculate the latest start time and the latest finish time for each task. Subtract the earliest finish time estimated for the project from the time it takes to complete the last task of the project. For example, if the earliest finish time for the project is 65 days, and the last task takes five days to complete, 60 days is the latest time the final task can be started without making the project miss its due date.

Continue calculating the latest start time for each task in the project. Consider all of the other tasks directly dependent upon each task in the project. Choose the one with the shortest time to use in calculating the latest start time for each task.

Determine the earliest start time of each task by taking the number of the latest finish time of the task immediately before it, as these are equivalent.

Determine the amount of slack in each task after you have completed both the forward and backward passes. Calculate slack by subtracting the earliest start time from the latest start time, or the earliest finish time from the latest finish time. Any task that has a slack time of zero is crucial to the project and is on the critical path.

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