Other Scheduling Methods - Construction Project Scheduling and Control - page 309

tie in with specific scheduling software. For example, Oracle's Primavera Risk Analysis 4
runs risk analysis by integrating with Primavera scheduling products. You can assign an
uncertainty to activity durations, costs, and resources, and then determine the proba-
bility of completing the project on time and within budget. Risk+ (ProjectGear, Inc.,
Tacoma, Washington) and @Risk (Palisade Corp., Newfield, New York) are compati-
ble with MS Project for calculating risk and probabilities based on PERT calculations.
Regardless of the product you use, you must ensure that it integrates with the schedul-
ing program you are using.
Some software companies call precedence network diagrams PERT . In most cases,
the displayed diagram is nothing but a precedence network applying the determinis-
tic CPM calculations. Because of this, many people in the construction industry have
beguntousetheterm PERT simply, and erroneously, to indicate a CPM logic diagram,
as compared with a bar chart. The author has even observed this error in terminol-
ogy in some owners' specifications, calling for “PERT reports” to be submitted. The
author strongly recommends refraining from this use of the term PERT .
GRAPHICAL EVALUATION AND REVIEW TECHNIQUE (GERT)
The graphical evaluation and review technique (GERT) is a network analysis tech-
nique that allows for conditional and probabilistic treatment of logical relationships
(i.e., some activities may not be performed) (Project Management Institute, 2000).
GERT is similar to PERT but considers both deterministic and probabilistic branch-
ing. It incorporates both in the network analysis. GERT allows additional branching
features that are not provided by CPM or PERT.
In deterministic branching, as shown in Figure 11.11a, we consider all activities
leaving the node. In other words, activities A, B, and C in Figure 11.11a will all take
place. In probabilistic branching, as shown in Figure 11.11b, we consider one branch
at any given time. For example, in Figure 11.11b, we consider either A or B. However,
the sum of the probabilities for each branch leaving a probabilistic branching must
equal 100% (Fair, 1994).
Furthermore, GERT allows looping, something both CPM and PERT do not
allow. For example, Figure 11.12 shows three possible outcomes following activity
A: activity B (with 30% probability), activity C (with 50% probability), or activity D
(repeat of activity A, with 20% probability). We can think of activity A as Shop Drawing
Submittal, activity B as Accepted As Is, activity C as Accepted with Changes, and
activity D as Rejected—Resubmit.
4 Originally called Pertmaster Professional Risk, created by Pertmaster Ltd., London, United Kingdom, and
then acquired by Primavera Systems, Inc., and renamed Primavera Pertmaster. With the recent acquisition of
Primavera by Oracle, Primavera Pertmaster was again renamed Oracle Risk Analysis.