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Defining the Critical Path in Microsoft Project

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The critical path is the longest path through the network, based on task duration, which defines the shortest amount of time in which the project can be completed. Tasks not on the critical path have slack, while tasks on the critical path have zero slack. Slack is the amount of time that a task can be delayed without impacting the project end date.

A project activity network looks like this:

Critical Path Diagram

There are a couple of other terms you may have seen used to refer to the critical path. This includes CPM, which is simply an acronym for the Critical Path Method. Also PERT (Program Evaluation and Review Technique), which is a process in which a probable outcome is evaluated based on three scenarios: a best-case, expected-case, and worst-case scenario. The outcome in question may be the duration of a task, its start date, or its finish date.

Calculating the critical path is nothing more than an estimate. Analysis of the critical path requires several assumptions, including the following:

◆ All tasks are known: If you forget to add some tasks and add them later, the original critical path may change. Most project plans are missing tasks when they are initially built.

◆ All links are accurate: A complicated plan with hundreds of tasks is likely to have some incorrect task dependencies defined.

◆ All estimates are accurate: Inaccurate estimates can cause the original critical path to change.

◆ Other non-critical paths may have small amounts of slack: A complicated activity path can have multiple noncritical paths. A change in any one of these paths can cause the original critical path to change.

Given the amount of assumptions we rely on in our analysis, the only time you know the actual critical path is after the project is finished.

Despite these potential problems, the critical path method gives the project manager a good indication of where to focus attention. The most important application of CPM is to realize a late task on the critical path will cause the project end date to change.

The following shows an example of calculating the critical path.

Path

Length of Path

Longest?

1-3-4-6-10

9 days

No

1-2-5-7-10*

12 days

Yes*

1-8-9-10

11 days

No

Note that tasks 3, 4, or 6 can be up to 3 days late without changing the critical path. These tasks each have three days of slack.

Also note that the tasks on the 1-2-5-7-10 path are on the critical path and have zero days of slack. If any task on the critical path is late, the project end date will be later.

◆ What would happen if Task 2 consumed five days instead of three days?

◆ What would the length of the 1-2-5-7-10 path become?

In this case would the 1-2-5-7-10 path still be critical?

Identifying the Critical Path Using Microsoft Project

The critical path is constantly calculated by Microsoft Project and can change as tasks are modified or updated. The Tracking Gantt Chart, described later in this chapter, includes the critical path by default.

You can also modify the standard Gantt Chart view to see which tasks are on the critical path in a project. The following exercise demonstrates how to do this.

  1. Go to File:Options to view the Project Options dialog, and then click Advanced in the sidebar. Scroll to the bottom and note the Tasks are critical if slack is less than or equal to option. Ensure that this is set to 0 days and click OK.

Project Option Dialog Box

Figure 3 The Project Options dialog box

  1. Apply the Schedule table to see the total slack for the tasks in this project. To do so, from View:Data click the Tables dropdown list and select Schedule. Note that the last column shows the total slack for each task.

 Filtering for Tasks with 0 Slack

  1. From View:Data select Critical from the Filter dropdown list. Only critical tasks in your project will now appear. Note: Look at the ID numbers of these tasks to see that not all tasks are displayed.

Only Tasks with 0 Slack Will Appear

 

Figure 5 Only critical tasks (0 slack) appear

  1. The Network Diagram view displays a node for tasks, along with schedule information and connecting arrows to show the relationship and sequence of activities. To see this view, from View:Task Views click the Network Diagram icon [Network Diagram Icon]. Your screen will resemble the following:

Network Diagram View

Figure 6 Network Diagram view

  1. Critical tasks can also be identified in the Gantt Chart. Reapply the Gantt Chart view, remove the Critical filter (so you again see all tasks in the project), and from Format: Bar Styles select the Critical Tasks checkbox.

Your screen will resemble the following:

Critical Tasks Identified in Red

Figure 7 Critical tasks identified in red in the Gantt Chart

Using an example project, you can link different tasks to each other to see how it affects the total slack of each task.

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