4. Service Activities for A Delta Jet During a 60 Minute
Layover
PERT and CPM
Network techniques
Developed in 1950’s
CPM by DuPont for chemical plants (1957)
PERT by Booz, Allen & Hamilton with the U.S. Navy,
for Polaris missile (1958)
Consider precedence relationships and interdependencies
Each uses a different estimate of activity times
Milwaukee General Hospital’s Activities and
Predecessors
Activity Description Immediate
Predecessors
A Build internal components -
B Modify roof and floor -
C Construct collection stack A
D Pour concrete and install frame A, B
E Build high-temperature burner C
F Install pollution control system C
G Install air pollution device D, E
H Inspect and test F, G
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5. F
A C
Start E H
B D G
Activity Description Immediate
Predecessors
A Build internal components -
B Modify roof and floor -
C Construct collection stack A
D Pour concrete and install frame A, B
E Build high-temperature burner C
F Install pollution control system C
G Install air pollution device D, E
H Inspect and test F, G
Latest Start and Finish Steps
Name
Activity
Earliest Earliest
Start ES EF
Finish
Latest
LS LF
Duration
Activity
Start Latest
Finish
Critical Path Analysis
Provides activity information
Earliest (ES) & latest (LS) start
Earliest (EF) & latest (LF) finish
Slack (S): Allowable delay
Identifies critical path
Longest path in network
Shortest time project can be completed
Any delay on critical path activities delays project
Critical path activities have 0 slack
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6. Earliest Start and Finish Steps
Begin at starting event and work forward
ES = 0 for starting activities
ES is earliest start
EF = ES + Activity time
EF is earliest finish
ES = Maximum EF of all predecessors for non-starting
activities
Latest Start and Finish Steps
Begin at ending event and work backward
LF = Maximum EF for ending activities
LF is latest finish; EF is earliest finish
LS = LF - Activity time
LS is latest start
LF = Minimum LS of all successors for non-ending
activities
Latest Start and Finish Steps
Name
Activity
Earliest Earliest
Start ES EF
Finish
Latest
LS LF
Duration
Activity
Start Latest
Finish
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7. We have 13 days for this project
Earliest Latest
Earliest Finish Start
Start A B CF 7
0 2 2 4 4
H H H
6 A 8 8 C10 10 13
2 2 3 Latest
Finish
We can begin the project as early as day 0 ---immediately
Task A costs 2 days, so the earliest day we can finish it is day 2
We must finish the project in day 13, so the latest finish day for task C is
day 13
Task C takes 3 days, so the latest time that we should begin it is day
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Critical Path for
Milwaukee General Hospital
F
A C
F
Start H
B D G
w
s sho
Arrow dence
prece ships
n
relatio
Earliest Latest
Finish Start
Earliest
Start
A C F
0 2 2 4 4 F 7
H H H
0 A 2 2 C4 10 13
2 2 3
E H
Slack=0 Slack=0 4 H 8 Slack=6 13 H 15
0 0 F
H
Start 4 8 15
0 0 4 13
0 2
B D Slack=0 G
Start 0 B 3 3 D7 8 G 13 Slack=0
H H H
1 4 4 8 8 13
3 4 5 Latest
Slack=1 Slack=1 Slack=0 Finish
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8. Gantt Chart
Earliest Start and Finish
Milwaukee General Hospital
1 2 3 4 5 6 7 8 9 10 1112 13 1415 16
A Build internal components
B Modify roof and floor
C Construct collection stack
D Pour concrete and install
frame
E Build high-temperature
burner
F Install pollution control
system
G Install air pollution device
H Inspect and test
Gantt Chart
Latest Start and Finish
Milwaukee General Hospital
1 2 3 4 5 6 7 8 9 10 1112 13 1415 16
A Build internal components
B Modify roof and floor
C Construct collection stack
D Pour concrete and install
frame
E Build high-temperature
burner
F Install pollution control
system
G Install air pollution device
H Inspect and test
PERT Activity Times
3 time estimates
Optimistic times (a)
Most-likely time (m)
Pessimistic time (b)
Follow beta distribution
Expected time: t = (a + 4m + b)/6
Variance of times: v = (b - a)2/6
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10. Obtaining the Probability
Standardized Normal Probability
Table (Portion)
Z .00 .01 .02
0.0 .50000 .50399 .50798 sZ =1
: : : :
2.0 .97725 .97784 .97831 .97725
2.1 .98214 .98257 .98300 mz = 0 2.0 Z
Probabilities in body
Variability of Completion Time for Noncritical
Paths
Variability of times for activities on noncritical paths must be
considered when finding the probability of finishing in a
specified time.
Variation in noncritical activity may cause change in critical
path.
Steps in Project Crashing
Compute the crash cost per time period. For crash costs
assumed linear over time:
(Crash cost − Normal cost
Crash cost per period =
(Normal time − Crash time )
Using current activity times, find the critical path
If there is only one critical path, then select the activity on this
critical path that (a) can still be crashed, and (b) has the
smallest crash cost per period. Note that a single activity may
be common to more than one critical path
Update all activity times.
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