Unformatted text preview: perceptions, not necessarily objective risk as
defined by the above equation. Perceptions of
consequences tend to grow faster than the consequences themselves—that is, several small
accidents are not perceived as strongly as one
large one, even if fatalities are identical.
There are difficulties in dealing with incommensurables, as for example, lives vs. dollars. mathematics of these trees is similar to that for decision
trees. The consequences of each accident sequence are
generally measured both in terms of direct economic
losses and in public health effects. (See sidebar on PRA
Doing a PRA is itself a major effort, requiring a
number of specialized skills other than those provided
by reliability engineers and human factors engineers.
PRAs also require large amounts of system design data
at the component level, and operational procedures
data. For additional information on PRAs, the system
engineer can reference the PRA Procedures Guide
(1983) by the American Nuclear Society and Institute of
Electrical and Electronic Engineers (IEEE).
Probabilistic Network Schedules.
network schedules, such as PERT (Program Evaluation
and Review Technique), permit the duration of each
activity to be treated as a random variable. By supplying
PERT with the minimum, maximum, and most likely
duration for each activity, a probability distribution can
be computed for project completion time. This can then
be used to determine, for example, the chances that a
project (or any set of tasks in the network) will be
completed by a given date. In this probabilistic setting,
however, a unique critical path may not exist. Some
practitioners have also cited difficulties in obtaining
meaningful input data for probabilistic network
schedules. A simpler alternative to a full probabilistic
network schedule is to perform a Monte Carlo simulation
of activity durations along the project's critical path.
(See Section 5.4.2.)
Probabilistic Cost and Effectiveness Models. These
models offer a probabilistic view of a project's cost and
View Full Document