What is Critical Path?
Critical Path Method is a process to identify the shortest time to finish a project and use the resources accordingly. Some of the non dependent tasks can be planned to process in parallel to reduce the project time.
The process steps are:
- Identify all the tasks that are required to complete the project
- Estimate the duration for each task
- Identify the dependencies
- Draw a flow diagram
- Put all the dependent activities in a workflow diagram
- Identify the longest time to complete dependent activities
- Longest stretch is the Critical Path.
The time taken to complete the Critical Path is the shortest time the project can be finished.
How Critical Path is Calculated?
It is the longest time in the network diagram to complete the Project. Read the Network diagram process below and you will be able to calculate.
Earliest start date. The earliest date that a task can be started in the project. When we define the dependencies between tasks, we have to calculate when a task can be started at the earliest. If there are any dependent tasks, then it means there is a constraint. This constraint will dictate the earliest start date.
Earliest finish date. The earliest date a task can be completed. First determine the dependencies. The dependent tasks will dictate when the task can be completed at the earliest. The constraints may be resources or other tasks.
Latest start date is the latest date a task can be started. It depends on the project expectations and other constraints.
Latest finish date is the latest date a task can be finished. It is determined by the project constraints and stakeholder expectations.
Once we have these information, it will give a clear picture of how to schedule tasks and to meet the project deadline.
Float or Slack:This will give an idea how long the task can be delayed. If the delay is threatening the deadline, then it has to be adjusted properly
Zero Float: For critical path, the tasks should have zero float.
Non-critical Path: When the tasks have some float, you can arrange the task in the non-critical path.
Crashing a project:
When you need to finish project quickly, you do the crashing.
You can add more resources in the tasks and complete the work quicker. Crashing needs more resources.
When a particular task is exceeding the planned time, you can crash the task so that you can avoid delays in the delivery of the final product.
Sometimes, when the team is required to work on another project, we tend to crash the present tasks and finish the project quickly.
Sample Critical Path Diagram
Critical Path construction
Constructing a Network Diagram
- List the activities
- Arrange the activities in a precedence table as shown below
Critical Path Table
| Activity | Predecessor | Duration (days) |
| A | 1 | |
| B | 2 | |
| C | B | 1 |
| D | 5 | |
| E | 5 | |
| F | A | 3 |
| G | C,F | 4 |
| H | G | 6 |
| I | D,E | 5 |
| J | H, I | 2 |
This table gives the Task Names/ Activity (A,B,C,D,E,F,G,H,I,J) with the predecessor and duration in days.
Draw a 3 columns by 2 rows diagram for each activity as shown below.
Write the Activity name (A,B…) as shown below. Enter the duration in the cell just below. Once all the cells are completed draw arrows connecting to the predecessors.
Forward Pass Method for Critical Path
Now let us start how to calculate and fill out those for all activities.
A, B, D, E – All activities have no predecessor or dependent activities.
They all can start anytime, so the ‘Earliest Start Date’ is ‘0’
Add the Earliest Start Date (ESD) and the duration to get Earliest Finish Date(EFD)
EFD=ESD+Duration
Activity A: EFD=0+1=1
Activity B: EFD=0+2=2
Activity D: EFD=0+5=5
Activity E: EFD=0+5=5
Now we will calculate for Other activities.
Activity C is dependent on B
C can start only after B is completed. B’s EFD is 2.
So C’s Earliest Start Date = 2+Duration
Activity C: EFD=2 + Duration= 2+1=3
Now we can calculate for F, G, H, J in a similar way. All these are dependent on the previous activity.3,4,6,2=1
F is dependent on A
G is dependent on F
H is dependent on G
J is dependent on H
Activity F: EFD = A’s Earliest Finish Date + F’s Duration
Activity F: EFD = 1+3= 4
Activity G: EFD = F’s Earliest Finish Date + G’s Duration
Activity G: EFD = 4+4= 8
Activity H: EFD = 8+6= 14
Activity J: EFD = 14+2= 16
Now Let’s finish the Late Start Date(LSD) and Late Finish Date(LFD)
Backward Pass Method
We have to go backward to calculate LSD & LFD
Start from the Last Activity (J)
Enter the LFD as EFD for J, 16
LFD-Duration = LSD
16-2=14; LSD for J IS 14
LFD for I is 14; LSD for I is 14-5=9;
LFD for E is 9; LSD for E is 9-5=4;
LFD for D is 9; LSD for D is 9-5=4;
LFD for H is 14; LSD for H is 14-6=8;
LFD for is 8; LSD for G is 8-4=4;
LFD for F is 4; LSD for F is 4-3=1;
LFD for A is 1; LSD for A is 1-1=0;
LFD for C is 4; LSD for C is 4-1=3;
LFD for B is 3; LSD for B is 3-2=1;
