CPM – Critical Path Method
CPM – Critical Path Method
In 1957, Dupont developed a project management method designed to address the challenge of shutting
down chemical plants for maintenance and then restarting the plants once the maintenance had been
completed. Given the complexity of the process, they developed the Critical Path Method (CPM) for
managing such projects.
CPM provides the following benefits:



Provides a graphical view of the project
Predicts the time required to complete the project.
Shows which activities are critical to maintaining the schedule and which are not.
CPM models the events and activities of a project as a network. Activities are depicted as nodes on the
network and events that signify the beginning or ending of activities are depicted as arcs or lines between
the nodes. The following is an example of a CPM network diagram based on the following Work
Breakdown Structure (WBS):
Landscape
Project
1.0
Design home
landscape
2.0
Put in lawn
2.2
Install
sprinkler
system
2.1
Purchase
lawn material
2.3
Plant grass
2.2.1
Identify
sprinkler
location
2.3.1
Remove
debris
2.2.2
Dig trenches
2.3.2.
Prepare soil
2.2.3
Install pipe
and hardware
2.3.3
Plant lawn
seed
3.0
Build Fence
2.4
Plant
shrubs
3.1
Aquire fence
material
3.2
Construct
fence
3.2.1
Mark fence
line and
posts
3.2.2
Install
posts
3.2.3
Install
fencing and
gates
3.2.4
Paint/stain
fence
= Summary Task or Milestone
2.2.4
Cover
sprinkler
system
= Work Package or Task
Sources: PMI PMBOK, Rita Mulcahy, NetMBA.com The Center for Management and Business Administration,
Inc. @ 2002-2004
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CPM – Critical Path Method
B
5 days
1 day
D
3 days
1 day
1 day
E
F
I
Finish
Start
A
7 days
C
2 days
H
3 days
J
2 days
L
M
3 days
5 days
N
5 days
K
G
2 days
5 days
Landscape Project Predecessor Table
WBS ID
Task ID
Task
1.0
A
Design home landscape
2.1/3.1
B
Acquire materials
2.2.1
C
ID sprinkler location
2.2.2
D
Dig trenches
2.2.3
E
Install pipe and hardware
2.2.4
F
Cover sprinkler system
2.3.1
G
Remove debris
2.3.2
H
Prepare soil
2.3.3
I
Plant lawn seed
2.4
J
Plant shrubs
3.2.1
K
Mark fence line
3.2.2
L
Install posts
3.2.3
M
Install fencing & gates
3.2.4
N
Paint/stain fence
Predecessor
A
A
G, C
B, D
E
A
G
G, F
H
G
B, K
L
M
Resources
Homeowner
Homeowner
Teens
Teens
Teens
Teens
Teens
Teens
Teens
Teens
Carpenter
Carpenter
Carpenter
Teens
Time Estimate
7
1
2
5
3
1
5
3
1
2
2
3
5
5
Steps in CPM Project Planning:
1.
2.
3.
4.
5.
6.
Specify the individual activities.
Determine the sequence of those activities.
Draw a network diagram.
Estimate the completion time for each activity.
Identify the critical path (the longest path through the network)
Update the CPM diagram as the project progresses.
1. Specify the individual activities.
From the Work Breakdown Structure, a listing can be made of all the activities in the project. This listing
can be used as the basis for adding sequence and duration information in later steps.
2. Determine the sequence of those activities.
Some activities are dependent upon the completion of others. A listing of the immediate predecessors of
each activity is useful for constructing the CPM network diagram.
3. Draw a network diagram.
Once the activities and their sequencing have been defined, the CPM diagram can be drawn. CPM
originally was developed as an activity on node (AON) network, but some project planners prefer to
specify the activities on the arcs.
Sources: PMI PMBOK, Rita Mulcahy, NetMBA.com The Center for Management and Business Administration,
Inc. @ 2002-2004
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CPM – Critical Path Method
4. Estimate the completion time for each activity.
The time required to complete each activity can be estimated using past experience or the estimates of
knowledgeable persons. CPM is a deterministic model that does not take into account variation in the
completion time, so only one number can be used for an activity’s time estimate.
5. Identify the critical path
The critical path is the longest-duration path through the network. The significance of the critical path is
that the activities that lie on it cannot be delayed without delaying the project. Because of its impact on
the entire project, critical path analysis is an important aspect of project planning.
The critical path can be identified by determining the following four parameters for each activity:




ES – earliest start time: the earliest time at which an activity can begin given that its predecessor
activities must be completed first.
EF – earliest finish time, equal to the earliest start time for the activity plus the time required to
complete the activity.
LF – latest finish time: the latest time at which an activity can be completed without delaying the
project.
LS – latest start time, equal to the latest finish time minus the time required to complete the
activity.
The slack or float time for an activity is the time between the earliest and latest start time, or between the
earliest and latest finish time. Slack is the amount of time that an activity can be delayed past its earliest
start or earliest finish without delaying the project.
The critical path is the path through the project network in which none of the activities have slack, that is,
the path for which LS=ES and LF=EF for all activities in the path. A delay in the critical path delays the
project. Similarly, to accelerate the project it is necessary to reduce the total time required for the
activities in the critical path.
6. Update CPM diagram
As the project progresses, the actual task completion times will be known and the diagram can be
updated to include this information. A new critical path may emerge, and structural changes may be
made in the network if project requirements change.
CPM limitations
CPM was developed for complex but fairly routine projects with minimal uncertainty in project completion
times. For less routine projects there is more uncertainty in the completion times, and this uncertainty
limits the usefulness of the deterministic CPM model. An alternative to CPM is the PERT (Program
Evaluation and Review Technique) project planning model, which allows a range of durations to be
specified for each activity.
Sources: PMI PMBOK, Rita Mulcahy, NetMBA.com The Center for Management and Business Administration,
Inc. @ 2002-2004
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