Operations and Supply Chain Management The Core

142 OPERATIONS AND SUPPLY CHAIN MANAGEMENT
exhibit 5.6
CPM Network for Computer Design Project
CPM Activity Designations and Time Estimates
Activity
Designation
Immediate
Predecessors
Time
(weeks)
Design
Build prototype
Evaluate equipment
Test prototype
Write equipment report
Write methods report
Write final report
A
B
C
D
E
F
G
–
A
A
B
C, D
C, D
E, F
21
5
7
2
5
8
2
C(7)
F(8)
A(21)
G(2)
B(5) D(2) E(5)
paths are 38, 35, 38, and 35 weeks. Note that this project has two different critical paths; this
might indicate that this would be a fairly difficult project to manage. Calculating the early
start and late start schedules gives additional insight into how difficult this project might be
to complete on time.
•
Early start schedule
A project schedule
that lists all activities
by their early start
times.
Late start schedule
A project schedule
that lists all activities
by their late start
times. This schedule
may create savings
by postponing
purchases of
material and other
costs associated with
the project.
Early Start and Late Start Schedules An early start schedule is one that lists all of the
activities by their early start times. For activities not on the critical path, there is slack time
between the completion of each activity and the start of the next activity. The early start
schedule completes the project and all its activities as soon as possible.
A late start schedule lists the activities to start as late as possible without delaying the
completion date of the project. One motivation for using a late start schedule is that savings
are realized by postponing purchases of materials, the use of labor, and other costs
until necessary. These calculations are shown in Exhibit 5.7. From this, we see that the
only activity that has slack is activity E. This certainly would be a fairly difficult project to
complete on time.
CPM with Three Activity Time Estimates
If a single estimate of the time required to complete an activity is not reliable, the best
procedure is to use three time estimates. These three times not only allow us to estimate
the activity time but also let us obtain a probability estimate for completion time for the
entire network. Briefly, the procedure is as follows: The estimated activity time is calculated
using a weighted average of a minimum, maximum, and most likely time estimate.
The expected completion time of the network is computed using the procedure described
above. Using estimates of variability for the activities on the critical path, the probability