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Page Title: Guideline 2.5-- Treatment of Uncertainty.
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produce benefits that accrue over time and (2) decision options that must be
repeated or extended to produce lasting benefits.
Guideline 2.5-- Treatment of Uncertainty. The RBP system should include a means to
address uncertainties in the prioritization results. Documentation should include a
discussion of how uncertainties affect the prioritization results. The sophistication of
the method chosen to address uncertainties and the level of effort devoted to
assessing the impact of uncertainties should be commensurate with the value of the
information as well as the scale of the effort.
Discussion. When addressing uncertainty, it is important to ensure inclusion of a
range of technical interpretations and viewpoints, and avoid forcing technical
consensus where it does not exist. An example helps to illuminate many of the
issues involved. Defense-in-depth is a common practice in limiting the risks of
hazardous operations that could threaten public health and safety. A prioritization
model that captures only known accident risks would value the prevention or
mitigation of these accidents but would accord little or no value to defense-in-depth.
To capture fully the value of defense-in-depth, it is necessary to undertake the difficult
task of evaluating all the uncertainties: some kinds of accidents may have been
missed, evaluations of the reliability of safety provisions may have been unduly
optimistic, or some accidents might proceed down paths not anticipated. Defense-in-
depth is intended to protect the public from risks that remain unmitigated due to just
such regulatory limitations.
The treatment of uncertainty may-- at its simplest-- be entirely qualitative. In many
cases, a mix of quantitative and qualitative methods are employed, with the qualitative
treatment reserved for those kinds of uncertainty that are difficult to evaluate
quantitatively, such as those that give value to defense-in-depth. It is also possible to
attempt a fully quantitative treatment of all uncertainties, though doing so for our
ignorance of all risk contributors inevitably entails a highly subjective approach. It is
not uncommon to be able to develop bounding estimates of what might have been
missed or distorted in a risk assessment, and these bounds are often useful in
illuminating the potential range of values that defense-in-depth strategies might
There are four key aspects of uncertainty in prioritization. A discussion of each is
provided below.
a. Variability and Uncertainty. Some inputs to the RBP system are predictable only in
a statistical fashion, i.e., there is a stochastic or random component. For example,
the RBP system may consider a risk due to accidents that affect public health and
safety. The number of individuals affected by such accidents is a variable quantity
(in the mathematical sense, a random variable that is described by a probability
distribution) that depends on the nature of the accidents considered, the weather
at the time of the accident, and individual response to such accidents. A common
way of expressing variabilities in risk assessments is through the use of
complementary cumulative distribution functions. In the above example, one could

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