Page 167 - DCAP601_SIMULATION_AND_MODELING

Unit 9: Simulation of a PERT Network (I)

Using these values, CPM calculates the longest path of planned activities to the end of the Notes
project, and the earliest and latest that each activity can start and finish without making the
project longer. This process determines which activities are “critical” (i.e., on the longest path)
and which have “total float”. A project can have several, parallel, near critical paths. An additional
parallel path through the network with the total durations shorter than the critical path is called
a sub-critical or non-critical path.

Notes It can be delayed without making the project longer). In project management, a
critical path is the sequence of project network activities which add up to the longest
overall duration. This determines the shortest time possible to complete the project. Any
delay of an activity on the critical path directly impacts the planned project completion
date (i.e. there is no float on the critical path.

These results allow managers to prioritize activities for the effective management of project
completion, and to shorten the planned critical path of a project by pruning critical path activities,
by “fast tracking” (i.e., performing more activities in parallel), and/or by “crashing the critical
path” (i.e., shortening the durations of critical path activities by adding resources).
Originally, the critical path method considered only logical dependencies between terminal
elements. Since then, it has been expanded to allow for the inclusion of resources related to each
activity, through processes called activity-based resource assignments and resource leveling. A
resource-leveled schedule may include delays due to resource bottlenecks (i.e., unavailability of
a resource at the required time), and may cause a previously shorter path to become the
longest or most “resource critical” path. A related concept is called the critical chain, which
attempts to protect activity and project durations from unforeseen delays due to resource
constraints.

Since project schedules change on a regular basis, CPM allows continuous monitoring of the
schedule, allows the project manager to track the critical activities, and alerts the project manager
to the possibility that non-critical activities may be delayed beyond their total float, thus creating
a new critical path and delaying project completion. In addition, the method can easily incorporate
the concepts of stochastic predictions, using the Program Evaluation and Review Technique
(PERT) and event chain methodology.
Currently, there are several software solutions available in industry that use the CPM method
of scheduling, see list of project management software. The method currently used by most
project management software is based on a manual calculation approach developed by Fondahl
of Stanford University.

Flexibility

A schedule generated using critical path techniques often is not realised precisely, as estimations
are used to calculate times: if one mistake is made, the results of the analysis may change. This
could cause an upset in the implementation of a project if the estimates are blindly believed, and
if changes are not addressed promptly. However, the structure of critical path analysis is such
that the variance from the original schedule caused by any change can be measured, and its
impact either ameliorated or adjusted for. Indeed, an important element of project
postmortem analysis is the As Built Critical Path (ABCP), which analyzes the specific
causes and impacts of changes between the planned schedule and eventual schedule as actually
implemented.

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