In spite of Critical Path being such an important method of managing a successful and timely delivery of project deliverables and as the name implies it being “critical”, it is surprisingly overlooked. How many times have you experienced a setback or delay in completing some of the project tasks whether due to any unexpected issues or dependencies or identified risks and things of that nature? To stay on track, critical path helps you mitigate the risk of falling behind the set & agreed schedule and also assists in managing those delays in a way that will cause no real negative effect on the firm project completion date.
Critical path method is used to calculate early & late start dates and finish dates without taking into consideration any resource constraints. Critical path method highlights that one path, out of various different paths in your schedule network diagram, that results in latest completion date of the project. Some of the tasks with a calculated float greater than “0” can be delayed but the tasks that fall under the critical path cannot be delayed. Any delay will cause increase in resources, schedule and related cost. Microsoft Project has an option for automatically displaying the critical path that should be monitored closely by any project manager. The rationale being that there could be delays on certain tasks but the maximum delay (without affection timeline) could only be what is permissible by the calculated slack or float. So when you notice your team is going slow on certain tasks and missing the completion date, you will need to figure out a way to restructure the dates for those delayed tasks in such a way that the project completion date is not affected. This is where critical path and slacks comes into picture.
We can easily find the critical path using Microsoft Project and also visually highlight the entire path in Gantt chart. Here are the sample tasks I created in MS Project; You can see the tasks highlighted in yellow(in task list) and red (in Gantt chart) are the tasks on critical path.
Now let’s assume Task 2 has been delayed to start on 5/29 instead of starting on 5/21 as originally planned. Since Task 2 has a slack of 9 days, we can delay the start date upto maximum 9 days before affecting the project completion date. See below on Gantt Chart how moving task 2 has no negative effect on the total time frame.
Calculating the slack manually will clear this concept from the ground level. Let’s say you have 9 WBS tasks in your project plan and the durations have been determined for each task. Next step is to determine the predecessors for each subsequent task. This forms a network schedule diagram which will be used for calculating the Critical Path. Start by designating the acronyms ES, EF, LS and LF to the 4 corners of the task box as shown in the diagram below.
Next step is to perform Forward Pass calculation to determine earliest start and earliest finish of each task. You start by assigning the value 0 to your first ES on Task 1. You will then add the duration for task 1 to the ES value (which is Zero for first task) and determine the EF. So EF for task 1 is 0 +2 = 2.
You will carry the values of EF =2 to ES of task 2 and ES of Task 3 since both the the successor tasks. Again you will add ES + Duration on Tasks 2 and 3 to get EF values.
Now before you proceed, notice that Task 4 has two predecessor tasks. So you will need to calculate the ES for both of the predecessor tasks (Task 2 and 6) and pick the highest EF value.
After calculating the Earliest Finish for Task 2 and Task 6, we will pick 16 as ES for Task 4 since that is the highest number out of 7 and 16.
Keep repeating these steps for each task until all ES and EF are calculated and determined.
Backward Pass needs to be performed for calculating the LF and LS and is followed in the opposite direction of forward pass, i.e, from Task 9 to Task 1.
Start by taking the Earliest Finish value of the last task and assign it to the Latest Finish of that new task. Now subtract duration from the Latest Finish to get Latest Start.
The LF of Task 9 is 37, so the Latest Start will be 37 – duration 3 = 34. Designate LS value to the LF of preceding task (from right to left perspective).
Keep performing the same steps until you reach Task 3. As you can see the values for LF of Task 3 can be carried over by both Task 5 and Task 6. Unlike forward pass method where you pick the highest values you will instead pick the lowest of the number. So the correct value carried over to LF of Task 3 will 10 (from 10 and 18 of Tasks 6 and 4 respectively)
The final step before we can determine the tasks falling on the critical pass is to calculate the float or slack. Float is the number of days a particular task can be delayed until the project completion is affected negatively.
To calculate float you will need to subtract Earliest Finish from Latest Finish (LF-EF) for each tasks. After calculating the float for all the tasks, you will note that there is a path where all the tasks have Zero float. That means there is no slack or delay allowed on any of those tasks. In our example, the critical path is in the order of Task 1, 3,5, 4,7 and 9.
Here is the video depicting each steps.