Other Scheduling Methods - Construction Project Scheduling and Control - page 292

INTRODUCTION
Although the critical path method (CPM) is the predominant method that is used for
scheduling construction projects, other methods can be used for particular types of
projects or in other industries. In this chapter, three of these methods are discussed.
( Note : Some of the information discussed in this chapter requires a basic knowledge
of statistics.)
PROGRAM EVALUATION AND REVIEW TECHNIQUE (PERT)
Definition
The program evaluation and review technique (PERT) is an event-oriented
network analysis technique that is used to estimate a project's duration when
individual activity duration estimates are highly uncertain. PERT applies the CPM
to a weighted-average duration estimate. PERT is considered a probabilistic, or
stochastic, method.
Background
In the 1950s, the U.S. Navy had a budget overrun and a schedule delay of as much
as 50% in the Polaris missile system project. The main problem was the lack of any
relevant historical data. The project team—the U.S. Navy (the owner), Lockheed Air-
craft Corporation (the prime contractor), and Booz Allen Hamilton (the management
consultant team)—launched a joint research effort to develop a tool to assist in the
planning of the Polaris project. The objective was to devise a method that would pre-
dict the completion date of a project with a certain likelihood using the theory of
probability. In 1958, this tool was developed under the name program evaluation and
review technique and later became known by its acronym, PERT.
The PERT system was adopted in its early days by the U.S. Department of
Defense (DoD) under the name program evaluation procedure (PEP) . After that,
PERT was modified several times and had different names: PERT I, PERT II,
PERT III, PERT/TIME, and PERT/COST. In June 1962, the DoD and the
National Aeronautics and Space Administration (NASA) adopted a uniform approach
to planning and controlling procedures for major weapon systems (Popescu and
Charoenngam, 1995).
Concept of PERT
Like the regular (deterministic) CPM, PERT uses logic networks to calculate the com-
pletion date of a project or the date of any other event in a schedule. In PERT, a
probability (likelihood) is associated with any event date. This probability depends
on uncertainty in the durations of the activities that lead to the desired event (e.g.,
project completion). PERT realizes that actual durations vary from those assigned, so
it attempts to compensate for this variation with a “time range” during which activity