Project Management: Critical path analysis

Posted: February 6, 2011 in Project Management
Tags: , , ,

As with Gantt Charts, Critical Path Analysis (CPA) or the Critical Path Method (CPM) helps you plan all tasks that must be completed as part of a project.

They act as the basis both for preparation of a schedule, and of resource planning. During management of a project, they allow you to monitor achievement of project goals. They help you to see where remedial action needs to be taken to get a project back on course. It took me a while to grasp this critical path concept not because it’s complicated but due to the lack of clarity around the available data.

What is critical path?

This seemingly complex topic is nothing more than a logical 2 dimensional tool you can use to determine the so called ‘important tasks’ or ‘hot tasks’ … nothing more than the tasks with the longest durations AND the tasks that fall on a crucial project path that is called ‘critical’ where if any of these ‘hot’ tasks is delayed, then the project outcome is at jeopardy.

Why is it so important?

It gives us a real-time snapshot on schedule lead & lag activity for risk response planning and an accurate representation of the most critical components of the schedule. It is the sequence of events for which, if any event is “improved”, the overall process will be “improved”. Here, “improved” is defined by reference to a specific performance measure.

The precedence relationship notation is included to improve the readability of the model, but the arrows are not used by the simulation. For simulation, the precedence relationships are established by the preconditions and postconditions of the events. A critical path is made up of activities that cannot be delayed without delaying the end date of the project.

How does one compute the critical path?

So what is the critical path and how do we determine it? Well, let’s first take a look at a network logic diagram for a simple sample project.

Each activity has a duration measured in weeks and the arrows show how each activity is depending on other activities to finish before they can start themselves. In the sample activity A must finish before C can start, and D can only start once C has finished etc. We can also see that activity C can only start once both A and B have finished.

From the diagram we can determine three separate paths:

Start – A – C – D – Finish: 8 weeks

Start – B – C – D – Finish: 9 weeks

Start – B – E – F – Finish: 7 weeks

The critical path is defined as the longest path in the diagram and in our example it is path B-C-D that is the critical path of 9 weeks. What’s so critical about it? If one of the activities on the critical path is delayed the entire project is delayed!

Example, if activity D is delayed 1 week, the project will be delayed with 1 week.

But if activity E is delayed 1 week, it will not delay the project because the path activity E is on will just be 8 weeks and still be done one week ahead of the BCD path.

So the critical path is made up of activities that cannot be delayed without delaying the finish of the entire project.

What happens if activity E suddenly is delayed 3 weeks? In that case the B-E-F path becomes the new critical path of 10 weeks and the finish of the project is delayed. The project manager must now determine how to handle this delay or accept it as the new critical path.

Will a project only have one critical path? A project can easily have more than one critical path and in that case the project manager must know all of them.

Hope you enjoyed this quick overview. To learn more please connect with us or subscribe to our blog.

Joe Zaarour, PMP


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