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Thursday, 25 January 2007 19:25

Critical Path

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The critical path is the longest path to completion in the network diagram. Activities on the critical path have no Slack or Float. The Project Time Management questions on the exam focus heavily on critical path method (CPM), and diagramming methods; the differences between these techniquees; and the appropriate circumstances for their use.

 

Critical path refers to the sequence of activities that must be completed on schedule for the entire project to be completed on schedule. In other words, if the end date for the project has slipped, it is because at least one activity on the critical path did not complete on time. It is important to understand the critical path sequence to know where you have schedule flexibility and where you do not. For instance, you may have a whole series of activities that end up running late, yet the overall project will still complete on time, because this series is off the critical path. On the other hand, all of your activities may be proceeding on schedule - except one. If that one activity is on the critical path, the entire project will start to fall behind schedule. If your project is falling behind, placing additional resources on non-critical activities will not result in the overall project completing earlier.


Critical Path Method - This is the most common approach to calculating when a project may finish. It uses a forward and backward pass to reveal which activities are considered critical. The critical path is used to determine which activities have no Slack (Float). Activities on the critical path may not be delayed; otherwise, the project end date will be pushed out.

 

Calculating the Critical Path
There is a manual method for calculating the critical path by looking at the earliest start and end dates for every activity starting at the beginning of a project. This is followed by starting at the end of the project and going backward, looking at the latest possible start and end dates for each activity that will allow the project to still complete on time. The difference between the earliest day an activity can start and the latest day it can start (and still finish on schedule) is the activity float. Once you have done this for all activities, look for the sequence of activities from start to end that have zero float. This is the critical path.

Fortunately, all project management software packages will calculate the critical path for you. All medium to large projects need to use a tool to manage the work plan. Take advantage of this automatic feature. For a small project, there may only be one major sequence of activities, and it should be easy to identify.

 

Free Float, Path Float and Total Float
When you are looking at the float in your project, you may come upon two terms - free float and total float. Free float is a term that is applied to individual activities. It refers to the amount of float in an activity before it delays the next activity. For example, if activity B can start up to five days after activity A completes without impacting the next activity, activity B has a free float of five. 


Total float refers to the total amount of float between all activities on all paths. If you have a lot of total float, then you usually have many more options as to how you allocate resources to achieve your due dates, and you have more flexibility if your project gets behind schedule. However, if total float on the project is low, that means you have more schedule risk, and much less flexibility. If the project starts to fall behind, you have a harder time reallocating resources, since if another path gets delayed, it may quickly turn into the critical path.

 

The Critical Path May Change
There are many sequences of activities on a project to get from the beginning to the end. There may, in fact, be multiple critical paths, if they all have no float and all lead to the same end date. Usually if there are multiple critical paths, they overlap for many of their activities 


given that there are many, many paths through the work plan; it's possible for the critical path to change. For instance, let's say we have a project with a path of 22 activities over 9 months. Let's assume that there is another path of work that includes 19 activities and takes 8 1/2 months (It has two weeks of path float). Now let's say that our client sponsor informs you that the project must be completed in eight months. First you would want to focus on accelerating the activities in the nine-month critical path. However, once the critical path is reduced to 8 1/2 months, the second critical path emerges that has the same 8 1/2 month timeline. Compressing the original critical path further will not make the project end earlier, because this second path is still going to take 8 1/2 months to complete. In this case, both paths must be accelerated. (Or perhaps some activities that are common to both can be accelerated.) 


The other way the path may change is if activities off the critical path get delayed to a point where a new path becomes critical. In the example above of a nine-month project, let's say that one of the activities on the 8 1/2 month path ends up taking an extra three weeks. Because there was only two weeks of float in the path (two weeks between the 8 1/2 month path and the 9 month critical path), this path will now become the critical path, and force the entire project to complete one week late. The project is not three weeks late because there was two weeks of slack in this second path. However, once the path was delayed two weeks, it became the critical path, and the third week of delay ended up causing the entire project to be delayed one week. 



Read 21373 times Last modified on Friday, 11 December 2009 01:26

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