Page 166 - DCAP601_SIMULATION_AND_MODELING

Simulation and Modelling

Notes 4. When the PERT/CPM charts become unwieldy, they are no longer used to supervise the
project.

9.3.3 Uncertainty in Project Scheduling

During project execution, however, a real-life project will never execute exactly as it was planned
due to uncertainty. It can be ambiguity resulting from subjective estimates that are prone to
human errors or it can be variability arising from unexpected events or risks. The main reason
that the Project Evaluation and Review Technique (PERT) may provide inaccurate information
about the project completion time is due to this schedule uncertainty. This inaccuracy is large
enough to render such estimates as not helpful.
One possibility to maximize solution sturdiness is to include safety in the baseline schedule in
order to absorb the anticipated disruptions. This is called proactive scheduling. A pure proactive
scheduling is a utopia, incorporating security in a baseline schedule that allows to cope with
every possible disruption would lead to a baseline schedule with a very large make-span.

Notes A second approach, reactive scheduling, consists of defining a procedure to react to
disruptions that cannot be absorbed by the baseline schedule.

9.4 Critical Path Method (CPM)

The critical path method is an algorithm for scheduling a set of project activities. It is an important
tool for effectual project management.
The Critical Path Method is a project modeling method developed in the late 1950s by Morgan
R. Walker of DuPont and James E. Kelley, Jr. of Remington Rand. Kelley and Walker related
their memories of the growth of CPM in 1989. Kelley attributed the term “critical path” to the
developers of the Program Evaluation and Review Technique which was developed at about the
same time by Booz Allen Hamilton and the US Navy. The precursors of what came to be known
as Critical Path were urbanized and put into practice by DuPont between 1940 and 1943 and
contributed to the success of the Manhattan Project.

CPM is usually used with all forms of projects, including construction, aerospace and defense,
software development, research projects, product development, engineering, and plant
maintenance, among others. Any project with mutually dependent activities can apply this
method of mathematical analysis. Although the original CPM program and approach is no
longer used, the term is usually applied to any approach used to analyze a project network logic
diagram.

Basic System

The essential technique for using CPM is to construct a model of the project that includes the
following:

1. A list of all activities required to complete the project (typically categorized within a work
breakdown structure),
2. The time (duration) that each activity will take to completion, and

3. The dependencies between the activities

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