Critical Path: An Extended Definition
®
by Kevin Aguanno, PMP , MAPM
C
ritical path is a term used in the field of project management to define a
sequence of tasks in a project wherein none of the tasks can be delayed
without affecting the final project end date. The sequence of tasks making
up the critical path starts with the first task in the project, and follows through to the
last task in the project. In effect, the timely completion of each task in the sequence
is critical to the timely completion of the project.
Knowing the critical path on a project, the project manager can apply additional
management techniques to the tasks in the critical path sequence to reduce the risk
of delays in the completion of those tasks and the overall project schedule. Critical
path is, therefore, a risk management technique.
History of the Critical Path Technique
The critical path concept evolved during the late 1950s on large defense research
projects in the United States of America. Meredith and Mantel (336-7) present two
different project scheduling methods developing the concept of a critical path at the
same time:
!
Critical Path Method. The Critical Path Method (CPM) was developed
by DuPont Inc. for use on construction-type projects with fixed project
end dates. CPM's main strength is the data it provides for making cost vs.
schedule tradeoff decisions. Today, the CPM is the most widely used
method for determining and managing the critical path on projects, and
is used in all industries.
!
Program Evaluation and Review Technique. This method, known by
the acronym PERT, was developed in 1958 by the consulting firm BoozAllen Hamilton and the defense contractor Lockheed Corporation for
use on the Polaris missile/submarine project. PERT uses probabilistic
methods to assess the likelihood that a project will finish by a given date.
This article was the first of
several in a series of
extended definitions of
common project
management terms and
techniques. The collected
set was to be used in a
textbook for beginning
students of project
management.
The publisher of the
textbook later canceled the
project, returning the
copyrights of the articles to
their authors.
Duncan (1996) promotes both the CPM and PERT techniques for determining the
critical path on a project. His Guide to the Project Management Body of Knowledge
(PMBOK Guide) published by the Project Management Institute, has become a
defacto baseline standard for project management knowledge and competency
assessments. Duncan describes the critical path techniques as being tools that are
“applicable to most projects most of the time, and that there is widespread
consensus about their value and usefulness” (1).
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2
Determining the Critical Path in a Project
The critical path identifies the sequence of tasks in which no single task can be
delayed without affecting the completion date of the sequence: in other words, a
sequence with no buffer (also called “float”). Figure 1 shows a traditional task
network diagram wherein the boxes represent tasks to be completed on a project.
The arrows connecting the boxes show the order in which the tasks must be
completed. For example, task B cannot start until task A has finished; this
dependency is shown with the arrow connecting the two tasks. Task D has two
arrows coming in to it; this means that both of the linked tasks (B and C) must be
complete before task D can start. The number in parentheses below each box
represents the number of days required to complete the task. A quick look at this
network diagram shows that there are three sequences of tasks (“paths”) that need
to be completed: the sequence A-B-D-E, the sequence A-C-D-E, and the sequence AC-E.
B
D
(3)
(4)
Beyond the PMP:
Advanced Project
Management Certification
Options.
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A
E
(1)
(1)
Available for $39.95 at:
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aguanno/book2.html
C
(1)
Figure 1: A sample project task network.
We can calculate the sequence with the longest duration by adding the numbers in
parentheses in each sequence. Figure 2 shows the calculations for the duration of
each sequence in the network. The sequence A-B-D-E has the longest duration at
nine days, making it the critical path. This means that any delay in a task in this
sequence will increase the total duration of the sequence and delay the final project
end date. On the other hand, if task C were delayed such that its actual duration
were now two days, task D would not have to start any later because it is also
dependent upon task B which would have not yet completed. Task C, therefore, is
not on the critical path.
Sequence
Calculation
A-B-D-E
1+3+4+1
Sequence
Duration
9 days
A-C-D-E
1+1+4+1
7 days
A-C-E
1+1+1
3 days
Figure 2: Calculating sequence durations.
©2002 Kevin Aguanno. All rights reserved.
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3
Limitations of the Critical Path Techniques
When a project manager provides extra attention to tasks on the critical path, it is
easy to lose sight of the risks inherent in the other tasks in the project. For example,
the completion of a task not on the critical path could be delayed so that its duration
becomes long enough to change the critical path of the project. Using our project
task network above, if the completion of task C required four days (instead of one
day), the task sequence A-C-D-E would be lengthened to a total of ten days, making
the sequence the new critical path on the project. Project managers should watch
for schedule risks in these non-critical tasks.
Another limitation of the critical path technique is that it highlights tasks at greatest
risk for causing schedule delays, but it does not identify the source of the schedule
risks. Alternate methods, such as the Critical Resource Path technique identified by
Aguanno (2002) calculate alternate critical paths that factor in the risks caused by
dependence on unique or constrained resources.
An Alternate Approach to Critical Path Techniques
101 Ways to Reward Your
Project Team for $20 (or
less!)
by Kevin Aguanno.
Other approaches to determining and managing schedule risk have emerged in
Available for $24.95 at:
recent years. For example, Goldratt developed the Critical Chain Method (1997)
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based upon his Theory of Constraints (1990) that addresses the problems of
aguanno/book1.html
constrained scheduling and process
optimization. Goldratt's Critical Chain
Works Cited
Method provides a practical means of
scheduling project task sequences (taking
Aguanno, Kevin. “Understanding the Critical Resource Path.” Book of
into account the common constraint of
Readings in Project Management. Hyderabad, India: Institute
relatively finite resource availability) to
of Chartered Financial Analysts of India - Center for
minimize risk in the project delivery
Management Research, 2002.
schedule. Patrick (1999 and 2001) has
- - -. “Using the Critical Resource Path.” Project Magazine 3.5 (2002). 13
refined the practical implementation of
F e b r u a r y 2 0 0 3 < h t t p : / / w w w. p r o j e c t m a g a z i n e
Goldratt's method and is a driving force
.com/may02/critical1.html>.
behind its growing adoption in the
- - -. “Using the Critical Resource Path to Reduce Project Risk.” Inside
corporate world.
Project Management 2.7 (2002): 1-4.
Why Use the Critical Path
Techniques?
While there are significant limitations in
using the critical path for schedule risk
management, the critical path techniques
provide a quick means of determining the
task sequence that is likely the source of
the greatest schedule risk in the project.
Widespread adoption of the techniques,
and their support in most project
scheduling software packages, have lead
to a growing understanding and
acceptance of critical path as a schedule
risk management technique among
executives. Critical path is a tool all
project managers should have in their
toolbox. a
Duncan, William R., ed. A Guide to the Project Management Body of
Newton Square, Pennsylvania:
Project
Knowledge .
Management Institute, 1996.
Goldratt, Eliyahu M. Critical Chain. Great Barrington, Massachusetts:
North River Press Publishing Corporation, 1997.
- - -. Theory of Constraints. Great Barrington, Massachusetts: North
River Press Publishing Corporation, 1990.
Meredith, Jack. R. and Mantel, Samuel J. Project Management: A
rd
Managerial Approach. 3 ed. New York: John Wiley & Sons,
1995.
Patrick, Francis S. "Critical Chain and Risk Management -- Protecting
Project Value from Uncertainty (Part 1)." Focused Performance
Nov. 2001.
13 Feb. 2003
<http://www.
Web Site
focusedperformance.com/articles/ccrisk.html>.
- - -. "Critical Chain Scheduling and Buffer Management . . . Getting Out
From Between Parkinson's Rock and Murphy's Hard Place."
Apr. 1999. 13 Feb. 2003
Focused Performance Web Site
<http://www.focusedperformance.com/articles/ccpm.html>.
©2002 Kevin Aguanno. All rights reserved.
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4
About the Author
As a Certified Senior Project Manager, Kevin Aguanno specializes in
managing complex software development projects. He has been
managing consulting, integration, and software development
projects in various industries since 1990.
Mr. Aguanno is certified by the Project Management Institute (PMI),
and is a member of both PMI and the Association for Project
Management (APM) in the United Kingdom.
Mr. Aguanno is the editor of Inside Project Management, an
element-K journal, and has written several books. His articles
appear regularly in various professional journals, and he speaks
internationally on project-management related issues.
Web site: http://www.mmpubs.com/aguanno
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Kevin Aguanno is available to speak at your corporate or association functions on general
topics ranging from project management, goal setting, and rewarding for performance to
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©2002 Kevin Aguanno. All rights reserved.
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