Re: hazard vs risk

["Fritz A. Seiler" <faseiler@nmia.com>]

Jo Anne Shatkin wrote:

> I am fascinated by the discussion of hazard versus risk, however do not feel

> any of the current opinions speak to my concern of whether or not one can

> qualitatively evaluate risk. ........ Is there any meaning for risk in
> qualitative terms?
>

A Comment on the Concepts of Hazard, and Risk,
and on the terms Qualitative and Quantitative Risk
sent to both RISKANAL and to RADSAFE
by Fritz A. Seiler and Joseph L. Alvarez

Dear Jo Anne,

     Risks and hazards are quite generally relating to the same
natural phenomenon or artificial construct.  For example, a
cumulus cloud is a hazard to aviation, as well as a reef is a
hazard to navigation, and ice on the road is a road hazard.
As long as these phenomena or artifacts are left alone, so that
they cannot interact with an object such as a plane, a ship, or
a car, there is no risk, although the hazard exists.
     Once one of the vehicles above occupies the same general
space at the same time, then there will be a risk of some
consequence or other.  Thus, once a pilot decides to fly through
the cumulus cloud, or a navigator lays out a course across the
reef, the corresponding part of the  hazard is transformed into a
risk.  It is important to realize that while the description of a
particular hazard may be quite complex mathematically, there is
no way to characterize a hazard numerically, it is just a model of
appropriate complexity.  Once a detailed course and speed are
given, however, the model can calculate the trajectory through
the cloud or the course across the reef and can evaluate the
ensuing risk.  In other words, a hazard describes the type of
conditions that may occur at different locations in the cloud or
across the reef, but it takes a well-defined, detailed space-time
scenario to quantify the risk of a particular consequence.
     By definition, therefore, risks are always quantitative, i.e.,
they are expressed in numbers.  The question of whether a risk
is given in qualitative or quantitative terms depends only on how
large its errors are.  If they are very large, a qualitative scale
may be used, for instance, by characterizing the risk as ‘high',
‘medium', ‘low', and ‘very low'.  Due to the large uncertainties,
this may then be called a qualitative risk assessment.  But we
need to remember that, with all its uncertainty, there is still a
coarse numerical judgment involved in classifying a risk as ‘low'
or ‘very low'.
     It is here that fuzzy sets can be used to advantage.  The
membership functions of the sets ‘small' and ‘very small' overlap
in a given region, say near 1E-3, and they are a direct expression
of our judgment to which class the value 1E-3 belongs, ‘small' or
‘very small'.  Note that this procedure is still a numerical
judgment, but is termed ‘qualitative' because of the large
uncertainties and the non-numerical designation of the classes.

We hope that this helps.

Fritz and Joe

*************************

Fritz A. Seiler, Ph.D.
Principal
Sigma Five Associates
P.O. Box 14006
Albuquerque, NM 87191-4006
Tel.    505-323-7848
Fax.    505-293-3911
e-mail: faseiler@nmia.com

**************************

Joe Alvarez, Ph.D., CHP
Auxier & Associates, Inc.
10317 Technology Dr., Suite 1
Knoxville, TN 37932
Phone (423)675-3669
FAX: (423)675-3677
Email: jalvarez@auxier.com

****************************

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