Calculating the Critical Path
The critical path is a key project management term. It identifies the tasks that cannot slip without affecting the project end date. Calculating the critical path is easy if you have software. However, understanding the theory behind the calculation can help show that there are other things just as important waiting to be discovered!
Prerequisites
It goes without saying that a full and complete Task List from a Work Breakdown Structure (WBS) is required. In order to calculate the critical path the estimates of durations need to be completed and accurate. The dependencies between the tasks also needs to have been correctly completed. If any of these are wrong, then our calculations will be incorrect.
The Forwards Pass
This short 5 minute clip explains how the earliest start times (EST) for every task is calculated:
I think it is useful to think logically rather than to try and remember the equation!
The Backwards Pass
The backwards pass reveals the latest start and finish (LST, LFT) times for each task. This short 4 minute clip explains:
Always use the LST as the EST for the finish task to make the calculations easier!
Network Terminology Float / Slack
The difference between the earliest and latest start times reveals the amount of Float or Slack on each task:
Any task with zero float is a critical task.
The Path of Least Float
The longest path through the project is the critical path. It represents the shortest time the project can be completed in. Usually, the critical path has zero float. A short 3 minute clip explains:
Negative float on the critical path is often called a hypercritical path, or supercritical path.
Always use the LST as the EST for the finish task to make the calculations easier!
Critical Path Example
Here is a worked example of critical path analysis:
There was no Lead or Lag in this example. Don’t forget to add lag going forward (and subtract it going backwards). You subtract lead going forward (and add it going backwards).
Summary
The critical path is the longest path through the project, and it has no float. However the process of finding the critical path has revealed how much float all of the other tasks have. The project manager can use this float to determine when other tasks happen, depending on resource availability, risk, and contingency.
Although usually calculated by a computer, the Project Manager has knowledge about risks, priorities, budgets, constraints and resources that can influence the final project schedule.
