Understanding critical path analysis
Critical Path Analysis is a powerful tool that helps you to plan and manage your projects.
Before we look at critical path analysis it is useful to define some terms. Let’s start with the critical path. This is the longest sequence of activities in a project plan that must be completed on time to ensure that the project is completed on time.
It is not possible to start another activity on the critical path until its preceding activity has been completed. For example, if we were decorating a room and we had to paint the skirting boards, the next task being to fit the carpets; both activities would be on the critical path and we wouldn’t be able to fit the carpets until the paint was dry (ideally!). Therefore, if we delay the painting by say, one day, the entire project will be delayed for a day. However, if we painted the skirting boards a day earlier, we would be able to fit the carpet a day earlier also (and assuming fitters were available). This would result in the project being completed a day early.
The critical path is very useful in helping to manage any project. Once we have identified the critical path, we can clearly see where effort cannot be compromised. If any of the activities on the critical path change, the end date of the project will be affected.
Critical Path Analysis
For all projects, the work needs to be broken down and we must identify and define all the activities that will be taking place. We can calculate how long each activity will take and once we know this, we can use this information to understand the duration of the project.
We can then complete a network schedule of activities as shown here.
The duration of each activity is listed above each node in the diagram. To calculate each path, we add the duration of each node to calculate its’ total duration. The critical path is the one with the longest duration.
In this project, there are 3 paths:
- Start then activity 1, then activity 3, then activity 4 then finish = 3+7+2 = 12
- Start then activity 2, then activity 3, then activity 4 then finish = 5+7+2 = 14
- Start then activity 2, then activity 5, then finish = 5+4 = 9
Remembering that the critical path is the pathway with the longest duration, the critical path for this project would be path number 2.
Having identified the critical path for the project, you can now calculate the float time for each activity. This is the amount of time an activity can slip before it causes a delay to your project.
To calculate the float time, start with the activities on the critical path. Any activities on the critical path cannot have a float, therefore we refer to this as zero. This is because, if any of those activities slips, the project will be delayed.
Therefore, we find the next longest path. Subtract its’ duration from the duration of the critical path and that will identify the float for each of the activities on that path.
Carry on doing the same for each subsequent longest path until each activities float has been determined. If an activity is on two paths, it’s float will be based on the longer path that it belongs to.
If we apply the float times to the previous project, we can see that because activities 2, 3, and 4 are on the critical path they have a float of zero.
The next longest path relates to activities 1, 3, and 4. Because activities 3 and 4 are also on the critical path, their float is also zero. For activity 1, the float will be the duration of the critical path minus the duration of this path. 14 – 12 = 2. Therefore, activity 1 has a float of 2.
The next longest path relates to activities 2 and 5. Because activity 2 is on the critical path it has a float of zero. Activity 5 has a float of 14 – 9, which is 5. Therefore, as long as Activity 5 doesn’t slip more than 5 days, it won’t cause a delay to the project.
Critical path analysis will enable you to manage your projects’ schedule effectively. For small projects that do not have many activities this is quite straightforward. However, this can become tedious with larger projects. Fortunately, there are many project management software tools available which can help you. Now that you understand the basics, you will be able to schedule your projects effectively and bring your projects in on time!
If you’ve enjoyed reading this article and would like to find out more, watch out for my upcoming blog on how to deliver an effective presentation. Or better still, why not join us at our project management workshop in September?
For more information contact Marie O’Donnell at Marie@professional-futures.com.