In order to fully understand the critical path and working with the critical path, a key term you should be familiar with is “slack." There are two types of slack: free slack and total slack. Free slack is specific to the task’s relationship with another task. In other words, free slack defines the amount of time that a task can be delayed before it affects or delays another task. Total slack is based on the total project—all tasks in the project. Total slack is the amount of time that a task can be delayed before it actually delays the project end date, or the completion of the project. By default, any task that is critical also has zero slack. In other words, any delay in that task will affect a delay in another task and in the project finish date. So a critical task has zero slack.
For example, Task A might have a relationship with Task B. Task B is the last task in the project. However, there might be two days of play time between the time Task A starts or finishes and the time Task B needs to start or finish. For example, assume that Task A has been assigned a duration of two days and Task B has been assigned a duration of four days. If Task A and Task B were to have a finish-to-finish relationship, they need to finish on the same day. In this scenario there is a two-day duration differential between Tasks A and B.
Because total slack is the amount of time that a task can be delayed before it delays the completion date of the project, that automatically gives Task A total slack of two days. This means that Task A remains noncritical only until it is delayed more than two days (or more than the total of its total slack time). Just because a task is noncritical today does not mean it is going to be noncritical tomorrow.In fact, you do not even have to display the critical path through the Gantt Chart Wizard in order to see how much slack you have built into your individual tasks in the project. You can view the amount of slack in the Detail Gantt view, shown in the first figure.
To display the Detail Gantt view, follow these steps:
Choose View and then More Views.
In the More Views dialog box, select Detail Gantt.
In addition to viewing the slack on the Detail Gantt view, you can also see the actual values for free and total slack in the Schedule table shown in the second figure. By default, the Entry table is assigned to the Gantt Chart view; however, you can just as easily assign the Schedule table in its place. All you have to do is choose View, Table, Entry, and then Schedule. Note that this graphic displays the same tasks as shown in the Detail Gantt graphic in the first figure to show how the slack in the Slack column matches the slack on the Gantt Chart.