Management Science : Project SchedulingPert_cpm.ppt

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© 2003 ThomsonTM/South-Western Slide
Slides Prepared by
JOHN S. LOUCKS
St. Edward’s University

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Chapter 10
Project Scheduling: PERT/CPM
 Project Scheduling with Known Activity Times
 Project Scheduling with Uncertain Activity Times
 Considering Time-Cost Trade-Offs

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PERT/CPM
 PERT
• Program Evaluation and Review Technique
•Developed by U.S. Navy for Polaris missile project
•Developed to handle uncertain activity times
 CPM
•Critical Path Method
•Developed by Du Pont & Remington Rand
•Developed for industrial projects for which
activity times generally were known
 Today’s project management software packages have
combined the best features of both approaches.

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PERT/CPM
 PERT and CPM have been used to plan, schedule, and
control a wide variety of projects:
•R&D of new products and processes
•Construction of buildings and highways
•Maintenance of large and complex equipment
•Design and installation of new systems

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PERT/CPM
 PERT/CPM is used to plan the scheduling of individual
activities that make up a project.
 Projects may have as many as several thousand
activities.
 A complicating factor in carrying out the activities is
that some activities depend on the completion of other
activities before they can be started.

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PERT/CPM
 Project managers rely on PERT/CPM to help them
answer questions such as:
•What is the total time to complete the project?
•What are the scheduled start and finish dates for each
specific activity?
•Which activities are critical and must be completed
exactly as scheduled to keep the project on schedule?
•How long can noncritical activities be delayed before
they cause an increase in the project completion time?

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Project Network
 A project network can be constructed to model the
precedence of the activities.
 The nodes of the network represent the activities.
 The arcs of the network reflect the precedence
relationships of the activities.
 A critical path for the network is a path consisting of
activities with zero slack.

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Example: Frank’s Fine Floats
Frank’s Fine Floats is in the business of building
elaborate parade floats. Frank and his crew have a new
float to build and want to use PERT/CPM to help them
manage the project .
The table on the next slide shows the activities that
comprise the project. Each activity’s estimated
completion time (in days) and immediate predecessors
are listed as well.
Frank wants to know the total time to complete the
project, which activities are critical, and the earliest and
latest start and finish dates for each activity.

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Immediate Completion
Activity Description Predecessors Time (days)
A Initial Paperwork --- 3
B Build Body A 3
C Build Frame A 2
D Finish Body B 3
E Finish Frame C 7
F Final Paperwork B,C 3
G Mount Body to Frame D,E 6
H Install Skirt on Frame C 2

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 Project Network
Start Finish
B
3
D
3
A
3
C
2
G
6
F
3
H
2
E
7

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Earliest Start and Finish Times
 Step 1: Make a forward pass through the network as
follows: For each activity i beginning at the Start
node, compute:
•Earliest Start Time = the maximum of the earliest
finish times of all activities immediately preceding
activity i. (This is 0 for an activity with no
predecessors.)
•Earliest Finish Time = (Earliest Start Time) + (Time
to complete activity i ).
The project completion time is the maximum of the
Earliest Finish Times at the Finish node.

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 Earliest Start and Finish Times
Start Finish
3 6
B
3
6 9
D
3
0 3
A
3
3 5
C
2
12 18
G
6
6 9
F
3
5 7
H
2
5 12
E
7

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Latest Start and Finish Times
 Step 2: Make a backwards pass through the network
as follows: Move sequentially backwards from the
Finish node to the Start node. At a given node, j,
consider all activities ending at node j. For each of
these activities, i, compute:
•Latest Finish Time = the minimum of the latest
start times beginning at node j. (For node N, this
is the project completion time.)
•Latest Start Time = (Latest Finish Time) - (Time to
complete activity i ).

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 Latest Start and Finish Times
Start Finish
3 6
6 9
B
3
6 9
9 12
D
3
0 3
0 3
A
3
3 5
3 5
C
2
12 18
12 18
G
6
6 9
15 18
F
3
5 7
16 18
H
2
5 12
5 12
E
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Determining the Critical Path
 Step 3: Calculate the slack time for each activity by:
Slack = (Latest Start) - (Earliest Start), or
= (Latest Finish) - (Earliest Finish).

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 Activity Slack Time
Activity ES EF LS LF Slack
A 0 3 0 3 0 (crit.)
B 3 6 6 9 3
C 3 5 3 5 0 (crit.)
D 6 9 9 12 3
E 5 12 5 12 0 (crit.)
F 6 9 15 18 9
G 12 18 12 18 0 (crit.)
H 5 7 16 18 11

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 Determining the Critical Path
•A critical path is a path of activities, from the Start
node to the Finish node, with 0 slack times.
•Critical Path: A – C – E – G
•The project completion time equals the maximum of
the activities’ earliest finish times.
•Project Completion Time: 18 days

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 Critical Path
Start Finish
3 6
6 9
B
3
6 9
9 12
D
3
0 3
0 3
A
3
3 5
3 5
C
2
12 18
12 18
G
6
6 9
15 18
F
3
5 7
16 18
H
2
5 12
5 12
E
7

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 In the three-time estimate approach, the time to
complete an activity is assumed to follow a Beta
distribution.
 An activity’s mean completion time is:
t = (a + 4m + b)/6
 An activity’s completion time variance is:
2 = ((b-a)/6)2
•a = the optimistic completion time estimate
•b = the pessimistic completion time estimate
•m = the most likely completion time estimate
Uncertain Activity Times

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Uncertain Activity Times
 In the three-time estimate approach, the critical path is
determined as if the mean times for the activities were
fixed times.
 The overall project completion time is assumed to have
a normal distribution with mean equal to the sum of the
means along the critical path and variance equal to the
sum of the variances along the critical path.

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Example: ABC Associates
 Consider the following project:
Immed. Optimistic Most Likely Pessimistic
Activity Predec. Time (Hr.) Time (Hr.) Time (Hr.)
A -- 4 6 8
B -- 1 4.5 5
C A 3 3 3
D A 4 5 6
E A 0.5 1 1.5
F B,C 3 4 5
G B,C 1 1.5 5
H E,F 5 6 7
I E,F 2 5 8
J D,H 2.5 2.75 4.5
K G,I 3 5 7

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 Project Network
E
Start
A
H
D
F
J
I
K
Finish
B
C
G
6
4
3
5
5
2
4
1
6
3
5

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 Activity Expected Times and Variances
t = (a + 4m + b)/6 2 = ((b-a)/6)2
Activity Expected Time Variance
A 6 4/9
B 4 4/9
C 3 0
D 5 1/9
E 1 1/36
F 4 1/9
G 2 4/9
H 6 1/9
I 5 1
J 3 1/9
K 5 4/9

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 Earliest/Latest Times and Slack
A 0 6 0 6 0 *
B 0 4 5 9 5
C 6 9 6 9 0 *
D 6 11 15 20 9
E 6 7 12 13 6
F 9 13 9 13 0 *
G 9 11 16 18 7
H 13 19 14 20 1
I 13 18 13 18 0 *
J 19 22 20 23 1
K 18 23 18 23 0 *

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 Determining the Critical Path
•A critical path is a path of activities, from the Start
node to the Finish node, with 0 slack times.
•Critical Path: A – C – F – I – K
•The project completion time equals the maximum of
the activities’ earliest finish times.
•Project Completion Time: 23 hours

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 Critical Path (A-C-F-I-K)
E
Start
A
H
D
F
J
I
K
Finish
B
C
G
6
4
3
5
5
2
4
1
6
3
5
0 6
0 6
9 13
9 13
13 18
13 18
9 11
16 18
13 19
14 20
19 22
20 23
18 23
18 23
6 7
12 13
6 9
6 9
0 4
5 9
6 11
15 20

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 Probability the project will be completed within 24 hrs
2 = 2
A + 2
C + 2
F + 2
H + 2
K
= 4/9 + 0 + 1/9 + 1 + 4/9
= 2
 = 1.414
z = (24 - 23)/(24-23)/1.414 = .71
From the Standard Normal Distribution table:
P(z < .71) = .5 + .2612 = .7612

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PERT/Cost
 PERT/Cost is a technique for monitoring costs during a
project.
 Work packages (groups of related activities) with
estimated budgets and completion times are evaluated.
 A cost status report may be calculated by determining
the cost overrun or underrun for each work package.
 Cost overrun or underrun is calculated by subtracting
the budgeted cost from the actual cost of the work
package.
 For work in progress, overrun or underrun may be
determined by subtracting the prorated budget cost
from the actual cost to date.

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PERT/Cost
 The overall project cost overrun or underrun at a
particular time during a project is determined by
summing the individual cost overruns and underruns
to date of the work packages.

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Example: How Are We Doing?
 Consider the following project network:
D
3
Start
A
9
I
4
H
5
E
4
B
8
Finish
C
10
J
8
F
4
G
3

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 Earliest/Latest Times
A 0 9 0 9 0 *
B 0 8 5 13 5
C 0 10 7 17 7
D 8 11 22 25 14
E 8 12 13 17 5
F 9 13 13 17 4
G 9 12 9 12 0 *
H 12 17 12 17 0 *
I 12 16 21 25 9
J 17 25 17 25 0 *

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 Activity Status (End of Week 11)
Activity Actual Cost % Complete
A $6,200 100
B 5,700 100
C 5,600 90
D 0 0
E 1,000 25
F 5,000 75
G 2,000 50
H 0 0
I 0 0
J 0 0

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 Cost Status Report
(Assuming a budgeted cost of $6000 for each activity)
Activity Actual Cost Value Difference
A $6,200 (1.00)x6000 = 6000 $200
B 5,700 (1.00)x6000 = 6000 - 300
C 5,600 (.90)x6000 = 5400 200
D 0 0 0
E 1,000 (.25)x6000 = 1500 - 500
F 5,000 (.75)x6000 = 4500 500
G 2,000 (.50)x6000 = 3000 -1000
H 0 0 0
I 0 0 0
J 0 0 0
Totals $25,500 $26,400 $- 900

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 PERT Diagram at End of Week 11
11
14
D
3
Start
11
11
A
0
12.5
16.5
I
4
12.5
17.5
H
5
11
14
E
3
11
11
B
0
Finish
11
12
C
1
17.5
25.5
J
8
11
12
F
1
11
12.5
G
1.5
Remaining
work (weeks)
Earliest Finish
Earliest Start

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 Corrective Action
Note that the project is currently experiencing a
$900 cost underrun, but the overall completion time is
now 25.5 weeks or a .5 week delay. Management
should consider using some of the $900 cost savings
and apply it to activity G to assist in a more rapid
completion of this activity (and hence the entire
project).

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Critical Path Method
 In the Critical Path Method (CPM) approach to project
scheduling, it is assumed that the normal time to
complete an activity, tj , which can be met at a normal
cost, cj , can be crashed to a reduced time, tj’, under
maximum crashing for an increased cost, cj’.
 Using CPM, activity j's maximum time reduction, Mj ,
may be calculated by: Mj = tj - tj'. It is assumed that its
cost per unit reduction, Kj , is linear and can be
calculated by: Kj = (cj' - cj)/Mj.

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End of Chapter 10

Management Science : Project SchedulingPert_cpm.ppt

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