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378 CHAPTER 9 PROJECT SCHEDULING: PERT/CPM

Figure 9.6 Souk Al Bustan Shopping Centre Project Network with Latest Start and Latest Finish Times
Shown in Each Node

E 5 6 F 6 10
1 5 6 4 6 10

A 0 5 D 5 8 G 10 24
5 0 5 3 7 10 14 10 24

C 5 9 H 9 21 I 24 26
Start Finish
4 8 12 12 12 24 2 24 26

B 0 6
6 6 12

One of the primary or no, slack. This activity cannot be delayed without increasing the completion time
contributions of PERT/ for the entire project. In other words, completing activity E exactly as scheduled is
CPM is the identification critical in terms of keeping the project on schedule. So, activity E is a critical activity.
of the critical activities.
The project manager will In general, the critical activities are the activities with zero slack.
want to monitor critical The start and finish times shown in Figure 9.6 can be used to develop a detailed
activities closely because start time and finish time schedule for all activities. Putting this information in
a delay in any one of tabular form provides the activity schedule shown in Table 9.2. Note that the slack
these activities will
lengthen the project column shows that activities A, E, F, G and I have zero slack. Hence, these activities
completion time. are the critical activities for the project. The path formed by nodes A–E–F–G–I is the
critical path in the Shopping Centre project network. The detailed schedule shown in
Table 9.2 indicates the slack or delay that can be tolerated for the noncritical activities
before these activities will increase project completion time.

The critical path
algorithm is essentially a Contributions of PERT/CPM
longest path algorithm.
From the start node to Previously, we stated that project managers look for procedures that will help
the finish node, the answer important questions regarding the planning, scheduling and controlling of
critical path identifies the
path that requires the projects. Let us reconsider these questions in light of the information that the critical
most time. path calculations have given us.
1 How long will the project take to complete?
Answer: The project can be completed in 26 weeks if each activity is completed
on schedule.
2 What are the scheduled start and completion times for each activity?
If the total time required
to complete the project is Answer: The activity schedule (see Table 9.2) shows the earliest start, latest
too long, judgement start, earliest finish and latest finish times for each activity.
about where and how to
shorten the time of 3 Which activities are critical and must be completed exactly as scheduled to
critical activities must be keep the project on schedule?
exercised. If any activity Answer: A, E, F, G and I are the critical activities.
times are altered, the
critical path calculations 4 How long can noncritical activities be delayed before they cause an increase in
should be repeated to the completion time for the project?
determine the impact on Answer: The activity schedule (see Table 9.2) shows the slack associated with
the activity schedule and
the impact on total each activity. So for example Activity H could take up to three weeks longer
project completion time. than planned before delaying completion of the whole project.

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