CPM-200: Principles of Schedule
Management
Lesson B: Critical Path Scheduling Techniques
Instructor
Jim Wrisley
IPM 2002 Fall Conference
PMI-College of Performance Management – Professional Education Program
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Lesson Objectives - 1
TLO #1: The student will understand the evolution
of network scheduling, be able to differentiate
between two (2) primary types of network
scheduling methods and understand the value
of resource loading the network.
◆ ELO #1: The student will recognize the evolution of PERT
Scheduling.
◆ ELO #2: The student will be able to recognize and the Precedence
Diagramming Method (PDM) type schedule.
◆ ELO #3: The student will recognize the value of resource loading a
schedule.
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Lesson Objective - 2
TLO #2: The student will understand how to create
and analyze a PDM type schedule.
◆ ELO #1: The student will be able to recognize and use the PERT
Formula for calculating durations.
◆ ELO #2: The student will be able to define the use of : Finish-to-Start,
Start-to-Start, Finish-to-Finish, relationships.
◆ ELO #3: The student will be able to define and calculate: critical path,
and total float on a PDM Schedule.
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Lesson Outline
◆ Reviewing History of Network Scheduling
◆ PERT Durations
◆ Present Three Major Types of Network
Schedule Estimating Techniques and
Associated Terminologies
◆ Demonstrate Network Computations
◆ Discuss the Importance of Resource Leveling
◆ Discussion
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Network Scheduling
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The History of Network Scheduling
PERT Scheduling
◆ U.S. Navy Special Projects
Office
◆ Polaris Missile Program
◆ 1957
Critical Path Method
◆ Remington-Rand and
Dupont
◆ Chemical processing
◆ 1957
- Activity-on-Node (AON) also known
as Precedence Diagramming Method
(PDM) --- Most Popular
- Activity-on-Arrow (AOA) also known
as Arrow Diagramming Method (ADM)
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Critical Path Method (CPM) – Estimating
Method
An Activity receives a Single “Point
Estimate” instead of the 3
required by PERT. Hence no
formula is needed.
Example: Estimated Duration = 17 days
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Program Evaluation Review Techniques
(PERT) – Estimating Method
The PERT Formula
Uses a 3 Point Estimate
Pessimistic + 4xMost Likely + Optimistic
6
Given:
P = 22 days
M= 17 days
O = 15 days
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22 + 68 + 15
105
=
6
=
17.5
6
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Monte Carlo Simulations – Estimating
Method
Monte Carlo Simulations
uses PERT’s 3 “Point
Estimates” as basis. But it
does not use the PERT
formula.
This statistical technique
utilizes a computer to
simulate the outcome of a
project. It will give you the
probability of completing
the project on any given
day!
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Pros and Cons of Estimating Methods
CPM: Easiest to use. Many
experts believe it is the least
accurate predictor.
PERT: More sophisticated
and requires more thought.
Many experts believe it is
more accurate than CPM.
Monte Carlo Simulations:
Most sophisticated. Many
experts believe it is the best
prediction technique
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Basic Scheduling Terms
Critical Path: Longest path through Project.
Has no slack.
Slack (Float): The amount of time an activity
can be delayed without delaying the project.
Lag: Inserted waiting time between tasks.
Estimating Methods: (CPM, PERT, Monte
Carlo)
Resource Leveling: Allows a more stable
number of resources to be used on the project.
Crashing The Schedule: Adding more resources
to the Critical Path
Fast Tracking: Doing more tasks in parallel.
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The Goals Of Project Management:
to make the most effective use
of available resources such as:
◆ People
◆ Equipment
◆ Facilities
◆ Materials
◆ Money
◆ Technology
so that project objectives
and goals can be achieved:
◆ Within budget
◆ On schedule
◆ To the desired quality
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WBS
Work Packages /
Activities
CONTROL ACCOUNT
WORK
PACKAGES
ACTIVITIES
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CALCULATING THE NETWORK
◆ Task Durations
◆ Forward Pass
◆ Backward Pass
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Task Durations (Estimates)
◆ Developed for each activity
◆ Developed by best
available authority
◆ Generally assumes normal
conditions (manpower,
equipment, calendar, etc.)
◆ CAUTION: - OverEstimating Tendencies
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Activity Types
DURATION
DRIVEN
Estimate based on amount of calendar
time necessary for task completion
EFFORT
DRIVEN
Estimate based on the amount of
work required to complete the activity
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PDM Network - A
L
H
5
4
A
4
D
3
I
M/S
0
Project
Start
2
B
3
3
K
M/S
4
Project
Completed
0
E
10
C
4
J
G
F
8
6
M
10
PROJECT LENGTH?
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Forward Pass
◆ From project start to finish, calculate the earliest
that each activity can start and finish according to
the logical sequence of work and the duration of
each activity
EARLY DATES
◆ Yields project duration
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PDM Network - A
14
1
4
5
7
001
004
A
D
4
17
0
M/S 0
000
Project
Start
H
L
5
4
3
1
3
4
002
005
B
E
3
13
2
12
14
15
16
18
19
22
009
010
011
I
J
K
3
30
M/S 29
014
Project
Completed
0
4
10
1
4
22
012
14
1
18
008
4
5
19
20
29
003
006
007
013
C
F
G
M
6
8
10
FORWARD PASS
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Backward Pass
◆ Working backward from project finish to start,
calculate the latest that each task must start and
finish in order to meet the end date.
LATE DATES
◆ Yields when the project must start to meet the
latest acceptable completion date.
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PDM Network - A
14
1
4
5
7
001
004
A
4
14
1
0
1
M/S 0
000
Project
Start
0
1
3
1
1
4
2
3
3
18
20
4
13
002
005
B
E
3
10
4
13
4
5
12
18
22
012
H
L
4
21
D
17
17
008
5
25
24
14
15
16
18
29
19
22
009
010
011
I
J
K
2
21
22
14
19
3
23
25
30
4
26
29
20
29
003
006
007
013
C
F
G
M
5
8
6
13
6
14
19
10
20
M/S 29
014
Project
Completed
0
30
29
29
BACKWARD PASS
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Why Calculate The Network?
◆ Establish ES & EF dates and project duration
◆ Calculate LF & LS dates based on project completion
◆ Defines “Float”
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Float
(Leeway Or Slack)
◆ Amount of time an activity can be delayed before it
impacts Project Completion
◆ Calculated by comparing LF to EF
1
4
001
A
4
14
= 13 days "Float"
17
◆ Also called Total/Path/Shared Float
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Total Float
EF
ES
1
2
3
4
5 6 7
8
9 10 11 12 13 14 15 16 17
A
A
Total Float for Activity A
LS
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Activity (Free) Float
◆ Amount of time an activity can be delayed before
it impacts any succeeding activity
6 days "Free Float"
5
7
14
17
004
008
D
H
3
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PDM Network - A
14
1
4
5
001
A
4
14
1
0
1
M/S 0
000
Project
Start
0
3
1
1
4
2
3
3
18
20
4
13
002
005
B
E
3
10
4
13
4
5
12
18
22
012
H
L
4
21
D
17
1
7
004
17
008
5
25
24
14
15
16
18
29
19
22
009
010
011
I
J
K
2
21
22
14
19
3
23
25
30
4
26
29
20
29
003
006
007
013
C
F
G
M
5
8
6
13
6
14
19
10
20
M/S 29
014
Project
Completed
0
30
29
29
Critical Path
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Critical Path
◆ Longest path of logically related activities through
the network which has the “least” Total Float.
◆ Defines project duration.
◆ Path:
B
E
G
M
3 + 10 + 6 + 10 = 29 days
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Resource Leveling
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Why We Resource Load Networks
◆ Assignment of resources (people, equipment, facilities,
materials, etc.) to each activity within the project
◆ Comparison of Needs vs. Availability
◆ Resulting Profiles (Histograms)
◆ Cumulative Plots (S-Curves)
◆ Other descriptive terms:
– Resource Allocation
– Resource Leveling
– Resource Scheduling
– Resource Smoothing
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We Develop A Staffing Plan
Because...
During Critical Path Method (CPM) schedule
development, the assumption has been that
unlimited manpower, material and equipment
are available
––– This OFTEN is not true –––
The availability of resources has a direct
bearing on the duration of each activity.
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CONTROL ACCOUNT
$
$
$
$
Total
$
$
$
$
$
$
$
$ $
A
ut
$1 $1 $2 $1 $2 $2 $2 $1 $1
om
at
ed
?
M
an
ua
l?
Hrs
Hrs
Hrs
Hrs
Hrs
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Hrs
Hrs
Hrs
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Resources-loaded Schedules
◆ Schedules that have required resources identified
to a specific schedule activity
ACTIVITY X
TITLE: Review Geology Report
EARLY
START 4/1
EARLY
FINISH 4/2
LATE
START 4/2
LATE
FINISH 4/3
DURATION: 2 days
TF = 1
Resources: 1 Hydrogeologist
1 Hydrogeologist Assistant
32 Hours
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PDM Network - A
18 May
1 May
4 May
7 May
004
A
D
4
H
L
5
4
30 May
4 Jun
23 May
24 May
29 May
18 May
21 May
1 May
3 May
4 May
17 May
002
005
B
E
3
11 Jun
22 May
24 May
25 May
31 May
010
011
I
J
K
2
30 May
31 May
18 May
25 May
3
4
1 Jun
6 Jun
5 Jun
11 Jun
11 Jun
10
1 May
1 May
3 May
4 May
4 May
7 May
16 May
29 May
11 Jun
006
007
013
C
F
G
M
8
7 May
8 May
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M/S
014
Project
Completed
0
17 May
003
4
2 May
5 Jun
009
1 May
0
31 May
3
18 May
M/S
000
Project
Start
24 May
012
9 May
001
23 May
008
10
6
17 May
18 May
25 May
29 May
11 Jun
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32
Network A
Activity
Duration
Total Float
Free Float
Resources
A
B
C
D
E
F
G
H*
I
J
K
L
M
4
3
4
3
10
8
6
4
2
3
4
5
10
13
0
1
13
0
1
0
7
7
7
7
7
0
0
0
0
6
0
1
0
0
0
0
7
7
0
24
6
12
3
40
16
30
4
14
18
8
20
20
215
*Duration Driven
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June
May
Activity
1
2 3
4
A
6
6 6
6
B
2
2 2
C
3
3 3
F
5 6
7 8 11
3
D
E
7 8 9 10 11 14 15 16 17 18 21 22 23 24 25 29 30 31 1 4
1 1 1
4
4 4 4
4
4 4 4 4
2 2 2
2
2 2 2 2
4
G
5
5
5 5
H
1
1
1 1
I
7
7
J
6 6
5 5
6
K
2
2 2
2
L
4 4
4 4
4
2 2
2
2 2
2 2
2 2
2
4 13 13 12 12 15 11 8 8
8
2 2
2 2
2 2
2
M
11 11 11 13 7 7 7
6
6 6 6 6
(
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= Late Finish)
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Total
by Day
- Net A
“NetResources
A” Resource
Profile
16
Resource Requirements
14
12
10
8
6
4
2
0
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
Project Days
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Why We Do Resource Analysis
◆ Determine resource requirements
(manpower projections, hire/fire strategies)
◆ Balance resource usage
◆ Consider time/resource tradeoff
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Total Float
EF
ES
1
2
3
4
5 6 7
8
9 10 11 12 13 14 15 16 17
A
A
Total Float for Activity A
LS
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May
Activity
1
2 3
4
A
6
6 6
6
B
2
2 2
C
3
3 3
F
7 8 9 10 11 14 15 16 17 18 21 22 23 24 25 29 30 31 1 4
5 6
7 8 11
3
D
E
June
1 1 1
4
4 4 4
4
4 4 4 4
2 2 2
2
2 2 2 2
4
G
5
5
5 5
H
1
1
1 1
I
7
7
J
6 6
5 5
6
K
2
2 2
2
L
4 4
4 4
4
2 2
2
2 2
2 2
2 2
2
4 13 13 12 12 15 11 8 8
8
2 2
2 2
2 2
2
M
11 11 11 13 7 7 7
6
6 6 6 6
(
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PDM Network - A
14
1
4
5
001
A
4
14
1
0
1
M/S 0
000
Project
Start
0
3
1
1
4
2
3
3
18
20
4
13
002
005
B
E
3
10
4
13
4
5
12
18
22
012
H
L
4
21
D
17
1
7
004
17
008
5
25
24
14
15
16
18
29
19
22
009
010
011
I
J
K
2
21
22
14
19
3
23
25
30
4
26
29
20
29
003
006
007
013
C
F
G
M
5
8
6
13
6
14
19
10
20
M/S 29
014
Project
Completed
0
30
29
29
Critical Path
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May
Activity
1
2 3
4
A
6
6 6
6
B
2
2 2
C
3
3 3
7 8 9 10 11 14 15 16 17 18 21 22 23 24 25 29 30 31 1 4
5 6
7 8 11
Total
Float
6 6 6 6
13
5
0
3
1
1 1
1 1 1
D
4
E
June
F
4 4 4
4
4 4 4 4
2 2 2
2
2 2 2 2
1
13
4
5
0
1
G
5
5
5 5
5 5
0
H
1
1
1 1
1 1
7
5
7
7
7 7
7
5
7
5
I
LOGIC A "DRIVES" D
J
6 6
D "DRIVES"
K
2 2
2
2 2
7
5
4 4
4 4
4
4 4
7
5
2 2
2
2 2
2 2
2 2
2
4 13 13 12 12 15 11 8 8
8
2 2
2 2
2 2
2
L
M
5
5 5
7
6
6 6 6
6 6 6 6
12 12 12 12 5 14 14 13 13 12 12 12
6
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6
2
H-L-I-J-K
11 11 11 13 7 7 7
6 6
6
0
8 8
(
= Late Finish)
Project Scheduling
40
"Net A" Resource Profile
14
Resource Requirements
12
10
8
6
4
2
0
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
Project Days
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‘S’ Curves
R
E
S
O
U
R
C
E
S
TIME-PHASED RESOURCE PLAN
(CUMULATIVE PLOT)
TIME
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May
Activity
1
2 3
4
A
6
6 6
6
B
2
2 2
C
3
3 3
F
7 8 9 10 11 14 15 16 17 18 21 22 23 24 25 29 30 31 1 4
5 6
7 8 11
3
D
E
June
1 1 1
4
4 4 4
4
4 4 4 4
2 2 2
2
2 2 2 2
4
G
5
5
5 5
H
1
1
1 1
I
7
7
J
6 6
5 5
6
K
2
2 2
2
L
4 4
4 4
4
2 2
2
2 2
2 2
2 2
2
4 13 13 12 12 15 11 8 8
8
2 2
2 2
2 2
2
M
11 11 11 13 7 7 7
6
6 6 6 6
(
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"Net A" Resource Curves
250
225
Cumulative Resources
200
175
150
125
Early Start
Late Start
Scheduled (Baseline)
100
75
50
25
0
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
Project Days
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FORECASTING RESOURCES
2
1
4
A
4
6
8
10
14
16
18
20
E
16
2
22
24
26
28
30
G
10
4
F
4
B
5
C
6
A
12
5
6
H
4
D
6
3
1 1 1 1
B
1 1
1 1
1
C
1 1
1 1
1 1
D
E
1 1
1 1
1 1
1 1
1 1
1 1 1 1
1 1
1 1 1 1 1 1
F
1 1 1 1
G
1 1 1 1 1 1 1 1 1 1
H
TOTAL
CUM
1 1 1 1
1 1 1 1 3 3 3 3 3 2 2 2 2 2 2 2 1 1 1 1 2 2 2 2 2 2 2 2 1 1
1 2 3 4 7 10 13 16 19 21 23 25 27 29 31 3334 35 36 37 39 41 43 45 47 49 51 53 54 55
USAGE TABLE
RESOURCES
3
2
1
TIME
R
E
S
O
U
R
C
E
S
PROFILE
50
40
30
20
10
CUMULATIVE (S) CURVE
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Should I Resource Load My
Schedule?
Pros:
¾ “Tight” integration of schedules and budgets
¾ Visibility of resource requirements by skill category
¾ Facilitates enterprise-wide resource planning
¾ Ease of “what if “ modeling during the planning phase
¾ Facilitates recovery planning
¾
¾
¾
¾
Cons:
Requires expert scheduling capabilities (software & personnel)
Over reliance on software tools instead of resource managers
Some resources are more critical than others
Benefits diminished if not done on an enterprise-wide basis
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Planning & Budgeting
Establish the Baseline - An Iterative 3-Step Process
1. Define The Work
Project Budget Base
Mgmt Res
2. Schedule The Work
$
Baseline
3. Allocate Resources
100
40
15
60
25
30
30
Time
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Additional Reading Resources
•Scheduling Guide for Program Managers: Defense
Acquisition University - 2001
•A Guide To Project Management Body of Knowledge:
Project Management Institute- 2000
• PMP Exam Prep- 2nd Edition: Rita Mulcahy, PMP
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