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376 CHAPTER 9 PROJECT SCHEDULING: PERT/CPM

Figure 9.4 Determining the Earliest Start Time for Activity H

A 0 5
Earliest finish time
5 for activity C is 9

C 5 9 H
Start
4 12
B 0 6
6 What are the earliest start and
finish times for activity H?
Earliest finish time
for activity B is 6

an activity time of 12 as shown in the node for activity H, the earliest finish time is
EF ¼ ES + t ¼ 9+12 ¼ 21. The ES ¼ 9 and EF ¼ 21 values can now be entered
in the node for activity H in Figure 9.4.
Continuing with this forward pass through the network, we can establish the
earliest start times and the earliest finish times for all activities in the network.
Figure 9.5 shows the Shopping Centre project network with the ES and EF values
for each activity. Note that the earliest finish time for activity I, the last activity in the
project, is 26 weeks. Therefore, we now know that the total completion time for the
project is 26 weeks.
We now continue the algorithm for finding the critical path by making a back-
ward pass through the network. Because the total completion time for the project is
26 weeks, we begin the backward pass with a latest finish time of 26 for activity I.
The latest finish time for an activity shows the latest time by which that activity must
be finished if it is not to delay completion of the whole project. Once the latest finish
time for an activity is known, the latest start time for an activity can be computed as
follows. Let:
LS ¼ latest start time for an activity
LF ¼ latest finish time for an activity

Figure 9.5 Souk Al Bustan Shopping Centre Project Network with Earliest Start and Earliest Finish Times
Shown for all Activities

E 5 6 F 6 10
1 4

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

C 5 9 H 9 21 I 24 26
Start Finish
4 12 2
EF = 26 for activity I; the
B 0 6 project can be completed
6 in 26 weeks.

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