Key Steps in the Critical Path Method
Source: www.smartsheet.com
There are six steps in the critical path method:
Step 1: Specify Each Activity
Using the work breakdown structure, you need to identify each activity (or task) involved in the project. This activity specification list should only include higher-level activities. When detailed activities are used, the critical path analysis may become too complex to manage and maintain.
A work breakdown structure breaks down projects into manageable sections.
The first step is to identify the main deliverables of a project. Then you can start breaking down the high-level activities into smaller chunks of work.
You can choose how to display your work breakdown structure. Some people use a tree structure, while others use lists or tables. An outline is one of the easiest ways to represent a work breakdown structure.
Step 2: Establish Dependencies (Activity Sequence)
Some activities will depend on the completion of others. Listing the immediate predecessors of each activity will help you identify the correct order. To correctly identify activities and their precedence, ask yourself these three questions for each activity on your list from step one:
- Which task should take place before this task happens?
- Which tasks should be finished at the same time as this task?
- Which tasks should happen right after this task?
Step 3: Draw the Network Diagram
Once you have identified the activities and their dependencies, you can draw the critical path analysis chart (CPA), known as the network diagram. The network diagram is a visual representation of the order of your activities based on dependencies.
Step 4: Estimate Activity Completion Time
Using past experience or the knowledge of an experienced team member, you must now estimate the time required to complete each activity. If you are managing a smaller project, you will most likely estimate time in days. If you are working on a complex project, you may have to measure time in weeks.
If you don’t feel comfortable using your best-guess estimates, you can use the 3-point estimation method, which is designed to put more weight on the most realistic timeframe.
In three-point estimation, you must come up with three-time estimates for every task, based on prior experience or best guesses. The estimation method is presented in formulas in order to calculate the time duration more accurately.
a = the best-case estimate
m = the most likely estimate
b = the worst-case estimate
These three values identify what happens in an optimal state, what is the most likely, and what happens in the worst case scenario.
Once you’ve identified these values, you can use them in two different formulas. The first is used to find the Weighted Average, which puts more weight on the “Most Likely” value. The formula is as below. E stands for Estimate, and the 4 and 6 represent the standard method to place more weight on the most realistic value.
E = (a + 4m + b) / 6
The second way of using these values is known as Triangular Distribution. The main difference is that this method doesn’t put more weight on the “Most Likely” value. The formula is as below. E stands for Estimate, and the 3 represents the standard method.
E = (a + m + b)/3
Step 5: Identify the Critical Path
There are two ways you can now identify the critical path. You can eyeball your network diagram and simply identify the longest path throughout the network -- the longest sequence of activities on the path. Be sure to look for the longest path in terms of longest duration in days, not the path with the most boxes or nodes.
You can also identify critical activities with the Forward Pass/Backward Pass technique, identifying the earliest start and finish times, and the latest start and finish times for each activity.
If you have multiple critical paths, you will run into network sensitivity. A project schedule is considered sensitive if the critical path is likely to change once the project begins. The more critical paths in a project, the higher the probability of a change in schedule.
Step 6: Update the Critical Path Diagram to Show Progress
As the project progresses, you will learn the actual activity completion times. The network diagram can then be updated to include this information (rather than continuing to use estimations).
By updating the network diagram as new information emerges, you may recalculate a different critical path. You will also have a more realistic view of the project completion due date and will be able to tell if you are on track or falling behind.
What is PERT?
Critical path method and PERT are often used in the same contexts and scenarios. Although they are similar, you should understand each concept and their differences.
A PERT chart, or activity-on-arrow diagram, is a visual representation of your project’s schedule, showing the sequence of tasks and which can be completed at the same time. A PERT chart is made with a lot of the same information that is used in the critical path method, like earliest and latest start dates, earliest and latest finish dates, and slack (or float) between activities.
But, the biggest difference between the critical path method and PERT is in time estimation. In the critical path method, time variance is not accounted for. You’re using best-guess estimates for completion time, and those times may change. With PERT, you’re putting more weight on the most realistic completion time.
Critical Path Method and PERT
You can use PERT in your critical path method in step four of the process, when you have to estimate activity completion times. The purpose of using PERT is to bias time estimates toward the most likely scenario and move away from unrealistically short timeframes.
To use PERT, you must estimate the shortest possible time each activity will take, the most likely length of time, and the longest time it might take if the activity lasts longer than expected.
With that information, you can use this formula when you have to estimate activity completion times in the critical path method (step four).
Shortest time + 4x likely time + longest time / 6 = expected activity completion time
How to Find the Critical Path on a PERT Chart
You can identify the critical path on a PERT chart because the chart includes the same information needed for the network diagram in the critical path method.
A PERT chart is drawn with circles for each activity, with the name of the activity and estimated duration in each circle. Arrows represent the paths that relate to dependencies.
To find the critical path on the PERT chart, first, identify how many paths you can take from start to finish. Then, add up the total duration of activities on that path. For example,
Path 1 duration: 12 days (task 1 and task 3)
Path 2 duration: 11 days (task 2 and task 3)
Path 3 duration: 10 days (task 4)
In this case, the critical path is task 1 and task 3 because it has the longest duration.
More information on https://www.smartsheet.com/critical-path-method