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Critical Path Method (CPM)
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Critical Path Method (CPM)
CPM is a network diagramming technique used to
predict total project duration.
A critical path for a project is the series ofactivities that
determines the earliest time by which the project can becompleted.
The critical path is the longest path through the network
diagram and has the least amount ofslack orfloat.
Slackorfloat is the amount oftime an activity can be
delayed without delaying a succeeding activity or the
project finish date.
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Calculating the Critical Path
Develop a good network diagram.
Add the duration estimates for all activities on
each path through the network diagram.
The longest path is the critical path.
Ifone or more ofthe activities on the critical path
takes longer than planned, the whole project
schedule will slip unless the project manager takes
corrective action.
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More on the Critical Path
The critical path does notnecessarily contain all the criticalactivities; it only accounts for time.
There canbe more than one critical path ifthe lengths oftwo or
more paths are the same.
The critical path can change as the project progresses.
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Network Representation
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The Network
Consider the following list ofactivities
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Which one is the correct network representation
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CPM Computations
We define the followings
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Forward Pass
A forward pass through the network diagramdetermines the early start and finish dates.
We visit all nodes from node 1 to the last one to
determine the earliest occurrence time
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Backward Pass
A backward pass determines the late start and finishdates.
We visit all nodes again, but backward to
determine the latest occurrence time
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Example
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Forward Pass
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Backward Pass
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Example 1. Find the critical path for the following
project
Activity Description Estimated
Duration (Days)
Predecessor
A Evaluate current technology
platform
2 None
B Define user requirements 5 AC Design Web page layouts 4 B
D Set-up Server 3 B
E Estimate Web traffic 1 B
F Test Web pages and links 4 C,D
G Move web pages to production
environment
3 D,E
H Write announcement of intranet
for corp. newsletter
2 F,G
I Train users 5 G
J Write report to management 1 H,I
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Example 2: Determining the Critical Path for Project X
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Example 6-5.2: Critical activities
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Extra
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Construction ofthe Time Schedule
Critical Vs Non critical activities
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Construction ofthe Time Schedule
Free float is the amount oftime an activity can be
delayed without delaying the early start ofany
immediatelyfollowing activities.
Total float is the amount oftime an activity canbe
delay
edfrom its earl
ystart without dela
ying theplanned project finish date.
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Example 6.5
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Red Flagging rule.
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Buffers and Critical Chain
A buffer is additional time to complete a task.
Critical chain scheduling removes buffers from individualtasks and instead creates:
A project buffer or additional time added before the projectsdue date.
Feeding buffers or additional time addedbefore tasks on thecritical path.
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Importance ofUpdating Critical Path Data
It is important to update project schedule
information to meet time goals for a project.
The critical path may change as you enter actual
start and finish dates.
Ifyou know the project completion date will slip,negotiate with the project sponsor.
