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Skip to 0 minutes and 1 secondSPEAKER: This video will demonstrate how to find the critical paths of a project. By using the critical path method, you can find the earliest and latest that an activity can start without delaying the overall duration of the project. In this example, we will look at a project for the preparation of a website for a corporate event. As you can see from the table, the project consists of eight activities. To apply the critical path method, we first need to draw a network diagram to show how the various activities are connected. The first phase of the critical path method is called the forward pass.

Skip to 0 minutes and 43 secondsThis lets you work out the earliest times that an activity can start and finish, otherwise known as the early starts and early finish dates. The basic rule in constructing a critical path diagram is an activity can't start until all of its predecessors have finished. In this example, we begin with Activity A, the kickoff meeting. This starts on Day 0 and lasts for one day. For Activity B, planning the content, and Activity C, designing the website, the earliest start date is Day 1, as this is the earliest date that Activity A, the kickoff meeting, can be completed by. Obviously, Activity D, writing the contents, can only start after Activity B, planning the content, has been completed.

Skip to 1 minute and 36 secondsNow let's look at Activity E, build the interactive elements. This activity has two predecessors, Activity B and Activity C. In order to find the earliest possible start date for Activity E, we look at the activities that have to come before it and then find the one with the highest value for its early finish date. So the highest early finish date here is the one for Activity B. This means that Day 6 becomes the early start date for Activity E. Using the same process, we can now fill in the table with the values for the earliest start and end dates. Activity H, closing the project, can't start before day 21 because it needs to wait to the end of Activity G.

Skip to 2 minutes and 22 secondsBy adding the durations of an activity to the earliest dates an activity can start, we can work out the earliest date that the project could be completed by. In this example, we now know that the project will last at least 22 days. Because we know the earliest date that the project can finish, we can use the critical path diagram to work out the latest time that we can start or finish an activity and still keep the project on track. This second phase of the critical path method is called the backward pass. To produce a backward pass, we need to start at the end of the project, in this case Activity H.

Skip to 3 minutes and 2 secondsTo calculate the late start of an activity, we subtract the length of time it takes to complete an activity from its latest finishing date. The late finish date in this case is 22 days. The duration of Activity H is one day. If we subtract that duration from the late finish date, we can work out that Day 21 is the latest day you can start Activity H and still complete the project on time. For Activity G, the late finish date is 21 and the duration is 5 days. This means that if the project is kept to schedule, the latest that Activity G can start is Day 16.

Skip to 3 minutes and 48 secondsFor a backward pass, the late finish date of one activity is the same as the earliest late start date of any following activities. Using the same process, we can now fill in the table with the values for the other late start and end dates. Finally, we get to Activity A. This is followed by Activities B and C. These both need to start on Day 1, therefore the late finish date of Activity A is 1. Now we can find the critical path. These are the activities that cannot be delayed without affecting the projects end date. They are easy to spot, as they have the same early and late start dates and the same early and late end dates.

Skip to 4 minutes and 32 secondsIn this example, there is one critical path, A, B, E, G, and H. Finally, we can calculate the amount of float available for the other activities. This is calculated by looking at the difference between the early start and the late start dates. For Activity F, the float is Day 5 minus Day 3, or 2 days. This means that Activity F can be delayed for up to two days without delaying the project. And with this information, we can now complete our table.

Applying the critical path method

In the previous step, the dates were calculated by looking at a figure, but if a project is bigger, you need to apply the critical path method (CPM) as shown in this video.

While you watch the video, replicate the calculation on a sheet of paper.

The next step gives you an opportunity to test your understanding of the CPM method. Don’t worry if you don’t get a question correct first time. The quiz in the next step will not affect your overall score for the course, and the point is to consolidate your understanding of the technique – so attempt the questions as many times as necessary to do so.

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This video is from the free online course:

Business Fundamentals: Project Management

The Open University