Page 164 - DCAP304_DCAP515_SOFTWARE_PROJECT_MANAGEMENT

Software Project Management

Notes One way to identify the critical path in the network diagram is to identify all possible paths
through the network diagram and add up the durations of the activities that lie along those
paths. The path with the longest duration time is the critical path. For projects of any size, this
method is not feasible, and we have to resort to the second method of finding the critical path—
computing the slack time of an activity.

Did u know? What are the uses of critical path method?
CPM can help you figure out:
 how long your complex project will take to complete

 which activities are “critical,” meaning that they have to be done on time or else the
whole project will take longer

Computing Slack

The second method of finding the critical path requires us to compute a quantity known as the
activity slack time. Slack time (also called float) is the amount of delay expressed in units of time
that could be tolerated in the starting time or completion time of an activity without causing a
delay in the completion of the project. Slack time is a calculated number. It is the difference
between the late finish and the early finish (LF – EF). If the result is greater than zero, the activity
has a range of time in which it can start and finish without delaying the project completion date,
as shown in Figure 8.9.
Figure 8.9: ES to LF Window of An Activity

Because weekends, holidays, and other non working periods are not conventionally considered
part of the slack, these must be subtracted from the period of slack.
There are two types of slack:
1. Free Slack: This is the range of dates in which an activity can finish without causing a delay
in the early schedule of any activities that are its immediate successors. Notice in (Figure
8.8) that activity C has an ES of the beginning of day 2 and a LF of the end of day 4. Its
duration is two days, and it has a day 3 window within which it must be completed
without affecting the ES of any of its successor activities (activity D and activity E). Therefore,
it has free slack of one day. Free slack can be equal to but never greater than total slack.
When you choose to delay the start of an activity, possibly for resource scheduling reasons,
first consider activities that have free slack associated with them. By definition, if an
activity’s completion stays within the free slack range, it can never delay the early start
date of any other activity in the project.
2. Total Slack: This is the range of dates in which an activity can finish without delaying the
project completion date. Look at activity E in Figure 8.8. Activity E has a free slack

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