The Internet Encyclopedia (Volume 3)

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ProjectMngmtTechn WL040/Bidgolio-Vol I WL040-Sample.cls June 19, 2003 16:49 Char Count= 0


118 PROJECTMANAGEMENTTECHNIQUES

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Figure 12: Network diagram and critical path in Project 2000.

will slip unless the project manager takes corrective
action.
People are often confused about what the critical path
is for a project or what it really means. Some people
think the critical path includes the most critical activi-
ties, but it is concerned only with the time dimension
of a project. Just because its name includes the word
critical does not mean that it includes all critical ac-
tivities. For example, Frank Addeman, executive project
director at Walt Disney Imagineering, explained in a
keynote address at the May 2000 PMI-ISSIG Professional
Development Seminar (http://www.pmi-issig.org) that
growing grass was on the critical path for building Dis-
ney’s Animal Kingdom theme park! This 500-acre park re-
quired special grass for its animal inhabitants, and some
of the grass took years to grow. Another misconception
is that the critical path is the shortest path through the
project network diagram. In some areas, for example,
transportation modeling, similar diagrams are drawn in
which identifying the shortest path is the goal. For a
project, however, each activity must be done to complete
the project. It is not a matter of choosing the shortest
path.

Program Evaluation Review Technique
Another project time management technique is the Pro-
gram Evaluation and Review Technique (PERT), a net-
work analysis technique used to estimate project dura-
tion when there is a high degree of uncertainty about
the individual activity duration estimates. PERT applies
the critical path method to a weighted average duration
estimate.
PERT uses probabilistic time estimates—duration esti-
mates based on using optimistic, most likely, and pessi-
mistic estimates of activity durations—instead of one spe-
cific or discrete duration estimate. Like the critical path
method, PERT is based on a project network diagram,
normally the PDM method. To use the PERT method, one

calculates a weighted average for the duration estimate of
each project activity using the following formula:
PERT weighted average=optimistic time+4X most
likely time+pessimistic time
The main advantage of PERT is that it attempts to ad-
dress the risk associated with duration estimates. PERT
has three disadvantages: it involves more work because
it requires several duration estimates, there are better
probabilistic methods for assessing risk (such as Monte
Carlo simulation), and it is rarely used in practice. In
fact, many people confuse PERT with project network di-
agrams because the latter are often referred to as PERT
charts.

Critical Chain Scheduling
A variation of critical path analysis that addresses the
challenge of meeting or beating project finish dates is
an application of the theory of constraints called critical
chain scheduling. The theory of constraints is based on
the fact that, like a chain with its weakest link, any com-
plex system at any point in time, often has only one aspect
or constraint that limits its ability to achieve more of its
goal. For the system to attain any significant improve-
ments, that constraint must be identified, and the whole
system must be managed with it in mind. Critical chain is
a method of scheduling that takes limited resources into
account when creating a project schedule and includes
buffers to protect the project completion date.
One can find the critical path for a project without con-
sidering resource allocation. For example, task duration
estimates and dependencies can be made without con-
sidering the availability of resources. In contrast, an im-
portant concept in critical chain scheduling is the avail-
ability of resources. If a particular resource is needed
full-time to complete two tasks that were originally
planned to occur simultaneously, critical chain schedul-
ing acknowledges that one of those tasks must be delayed
until the resource is available or another resource must
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