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2
Understanding Hazards and Risks
Throughout recorded history people have engaged in hazardous
activities, and governments have taken action to control some of those
activities in the public interest. But in recent times the hazards
of greatest concern, and knowledge about them, have changed in
ways that make informed decisions harder to reach. Once the focus
was simply on the presence or absence of danger. If a food was
"adulterated," if water was determined to be "impure," if a bridge or
dam was declared "unsafe," or if a workplace was "dangerous," action
was called for. When people called on government to take action,
they wanted simple, clear-cut measures: ban sale of the food, supply
pure water, condemn the bridge, eliminate the workplace hazard.
But with increased understanding of the nature of the choices, it
has become harder to maintain a simple view. Responsible decision
makers need to know more about the alternatives than that one of
them is hazardous.
In this chapter we outline the many kinds of knowledge a well-
informed decision requires and the ways in which this knowledge is
often incomplete and uncertain. We show how, under such condi-
tions, the judgments of both experts and nonexperts can be affected
by preexisting biases and cognitive limitations and how human val-
ues and concerns inevitably enter into the analytic process. These
factors often lead experts to disagree with each other and with non
30
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UNDERSTANDING HAZARDS AND RISKS
31
experts about the significance of risks, even when the facts are not
in dispute.
TOWARD QUANTIFICATION OF HAZARDS
One reason decision makers need more knowledge is that it has
become clear that eliminating one danger can create a new one. To rid
the water supply of organisms that cause typhoid and other infectious
diseases, water has been chlorinated since early in this century. This
action resulted in chemical reactions in the water that produced
chloroform and other carcinogenic chlorinated hydrocarbons. To
choose between the dangers, one must answer difficult questions:
Which danger is more worth avoiding? How much decreased danger
from typhoid is enough to justify a certain amount of increased
danger of cancer? Experts agree that there will be fewer deaths from
chIorination-induced cancer than there once were from typhoid, but is
that enough information to make a decision? It may be important to
consider that typhoid and cancer are very different kinds of dangers.
Typhoid is an acute disease and cancer is a chronic one; typhoid is
much more treatable; and there are alternatives to chlorination for
preventing it, although the alternatives also present hazards, as yet
poorly understood.
Society is faced with many choices that trade one danger for
another and that raise similar questions. For instance, regulated
commercial canning of food reduced the danger of botulism compared
with home canning, but the use of lead solder in "tin" cans introduced
a toxin not present in home canning jars. Lighter automobiles use
less fuel and generate less air pollution, but in a collision with an
older, heavier vehicle they are more dangerous to their occupants.
Societal choices also involve the benefits associated with hazards
and the costs of hazard reduction. Industries that pollute air and
water also provide jobs and profits; before requiring pollution con-
trols, public officials usually want to consider the probable effects
of the available options on those benefits. Cities may install traffic
lights to reduce fatalities and injuries, but officials usually want to
consider whether this is the best way to spend scarce revenues. Thus
decision makers want good estimates of how much each alternative
will reduce hazards so that they can judge the potential benefits
against the potential costs.
Decision makers need detailed knowledge because it has become
clear that making the world safer for most people can make it more
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IMPROVING RISK COMMUNICATION
dangerous for some. Pesticides and herbicides have helped make
wholesome food more available and have helped improve the diets of
low-income consumers, but they expose agricultural workers to haz-
ardous chemicals and can be a significant polluter of water supplies.
The total danger to society may have decreased greatly, but that
knowledge may be of no comfort to farm workers. Nuclear power
offers some people the benefit of cleaner air but may expose different
people to radioactivity in the event of an accident. How is society
to weigh small benefits to many against what are sometimes larger
dangers for a relative few?
Decision makers need detailed knowledge for another reason as
well: the hazards of greatest concern today are more difficult to
observe and evaluate than the major hazards of the past. Half a
century ago most of the major health and safety hazards were of
immediate onset: accidents, bacterial infections, poisonings, and the
like. Most of the hazards that are now controversial are of delayed
onset, sometimes not being evident for decades after exposure and
sometimes affecting only the offspring of those who were exposed. It
can be hard to know what the hazards of a substance or activity are
before a generation of experience has accumulated.
To make informed choices, it helps to look carefully and ana-
Tytically at the hazards each alternative entails. It is important to
develop quantitative knowledge: How much cancer might be caused
by chlorinating water? How much pesticide are farm workers ex-
posed to? For this kind of analysis, some conceptual distinctions are
useful. The most basic of these is between "hazard" and "risk." An
act or phenomenon is said; to pose a hazard when it has the potential
to produce harm or other undesirable consequences to some person
or thing. The magnitude of the hazard is the amount of harm that
may result, including the number of people or things exposed and
the severity of consequence. The concept of risk further quantifies
hazards by attaching the probability of being realized to each level of
potential harm.) Thus an area that experiences a severe hurricane
once in 200 years faces the same hazard but only one-tenth the risk
of a similar area that experiences an equally severe hurricane once in
20 years. The concept of risk makes clear that hazards of the same
magnitude do not always pose equal risks.
Risks of the same magnitude do not always pose equal concerns,
either. Most quantitative measures of risk combine the undesirability
of a hazard and its probability of occurrence into a single summary
measure. Use of such summary measures can simplify large amounts
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UNDERSTANDING HAZARDS AND RISKS
33
of data but can be unsatisfying to people who want to consider
different kinds of injuries or deaths separately because, for instance,
they believe that certain types of individuals are worthy of special
protection or that certain types of injuries or illnesses are especially to
be avoided. Some ways of characterizing risk take such concerns into
account. These involve calculating separate risk estimates for each
hazardous effect, giving heavier weight to qualitative characteristics
of risk (e.g., Fischhofl: et al., 1984; Okrent, 1980) and using explicit
measures of values and risk attitudes (Regina, 1968~.
KNOWLEDGE NEEDED FOR RISE DECISIONS
What kinds of knowledge must be collected so that the process
of communication will be an informed dialogue leading to reasonable
choices? Understanding the risks is not enough, because organi-
zations and individuals never choose between risks. Rather, they
choose between options, each of which presents some risks. Each also
presents benefits, which are as crucial to the choices as the risks are.
Understanding risks can be difficult, but understanding the benefits
of a set of decision alternatives can be as difficult. Both kinds of
knowledge are needed for an informed choice.
This section outlines the many kinds of relevant knowledge. It
summarizes four kinds of knowledge decision makers need: (1) about
risks and benefits associated with a particular option, (2) about alter-
native options and their risks and benefits, (3) about the uncertainty
of the relevant information, and (4) about the management situation.
Formation About the Nature of Risks and Benefits
"Risk assessment" is the term generally used to refer to the
characterization of the potential adverse effects of exposures to haz-
ards. Risk assessment therefore addresses the questions listed below.
"Benefit assessment," a term not commonly used, addresses many
sirn~lar questions. Some benefit questions are mentioned below, in
parentheses.
1. What are the hazards of concern as a consequence of a sub-
stance or activity? What environments, species, individuals, or organ
systems might be harmed? How serious is each potential conse-
quence? Is it reversible? (What are the benefits associated with a
substance or activity? Who benefits and in what ways?)
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IMPROVING RISK COMMUNICATION
2. What is the probable exposure to each hazard in total number
of people or valued things? How do the exposures cumulate over
time? A single exposure over a short period of time can have ejects
different from those due to exposure to the same amount of a hazard
in several episodes or chronically at low levels over a longer period of
time. (How many people benefit? How long do the benefits last?)
3. What is the probability of each type of harm from a given
exposure to each hazard? How potent is the hazardous substance or
activity at the relevant exposures? What is the relation of exposure
or "dose" to response? (What is the probability that the projected
benefits will actually follow from the activity in question? What
events might intervene to prevent those benefits from being received?
What are the probabilities of these events?)
4. What is the distribution of exposure? In particular, which
groups receive a disproportionate share of the exposure? (Which
groups get a disproportionate share of the benefits?)
5. What are the sensitivities of different populations of individ-
uals to each hazard? What is the appropriate estimate of harm for
highly sensitive populations that bear a significant proportion of the
overall risk? What are those populations, where are they located,
and what proportion of the total risk do they bear?
6. Now do exposures interact with exposures to other hazards?
Sometimes one exposure can make people more sensitive to another
hazard- a synergistic effect-and, occasionally, exposure to one haz-
ard may decrease sensitivity to another a blocking eject. What is
known about such effects?
7. What are the quantities of the hazard? For instance, do those
exposed have an option to reduce or eliminate their exposure (and at
what cost)? Would harm come to exposed people one at a time or as
a mass, in a potential catastrophe? Is the hazard deadly or not? Does
the harm take the form of accident or illness, acute or chronic disease,
damage to the young or the old, to the living or the unborn? If the
hazard is an illness, is it treatable? Is it a dread illness, such as cancer,
or one that creates less of an emotional reaction? Table 2.1 lists
qualities of risk that make a difference in most people's judgments.
(What are the qualities of the benefits? Do they appear as increased
income, saved time, physical comfort, improved health, more stable
ecosystems, more beautiful surroundings, improved welfare for low-
income people or the elderly, or in other forms?)
8. What is the total population risk, taking into account all of
the above? To arrive at such an estimate, one must somehow calcu
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UNDERSTANDING HAZARDS AND RISKS
TABLE 2.1 Qualitative Factors Affecting Risk Perception and Evaluation
35
Factor
Conditions Associated
with Increased Public
Concern
Conditions Associated
with Decreased Public
Concern
Catastrophic
potential
Familiarity
Underst ending
Controllability
(personal)
Voluntariness of
exposure
Effects on children
Effects manifestation
Effects on future
generations
Victim identity
Dread
Trust in institutions
Media attention
Accident history
Equity
Benefits
Reversibility
Origin
Fatalities and injuries
grouped in time and
space
Unfamiliar
Mechanisms or process
not understood
Uncontrollable
Involuntary
Children specifically
at risk
Delayed effects
Risk to future
generations
Identifiable victims
Effects dreaded
Lack of trust in
responsible institutions
Much media attention
Major and sometimes
minor accidents
Inequitable distribution
of risks and benefits
Unclear benefits
Effects irreversible
Caused by human actions
or failures
Fatalities and injuries
scattered and random
Familiar
Mechanisms or process
understood
Controllable
Voluntary
Children not
specifically at risk
Immediate effects
No risk to future
generations
Statistical victims
Effects not dreaded
Trust in responsible
institutions
Little media attention
No major or minor
accidents
Equitable distribution
of risks and benefits
Clear benefits
Effects reversible
Caused by acts of
nature or God
NOTE: In selecting risks to be compared, it is helpful to keep these distinctions
in mind. Risk comparisons that ignore these distinctions (e.g., comparing voluntary
to involuntary risks) are likely to backfire unless appropriate qualifications are
made.
SOURCE: Covello et al., 1988.
late a summation across different types of harm, people of different
sensitivities, and exposures to the hazard in different amounts and in
combination with various other hazards. (What is the total benefit?)
~fonnation on Alternatives
The term "risk control assessment" may be used to describe
the activity of characterizing alternative interventions to reduce or
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IMPROVING RISK COMMUNICATION
eliminate a hazard. More generally, decision makers need responses
to questions such as the following about all the alternatives to any
option under consideration:
1. What are the alternatives that would prevent the hazard in
question? Some involve the choice of alternative processes or sub-
stances, while others involve action that might prevent or reduce
exposure, mitigate the consequences, or compensate for damage.
2. What are the risks of alternative actions and of a decision
not to act? How are these risks distributed? Since there are an
infinite number of alternatives, it is possible to assess only a few,
but a complete analysis should examine those alternatives being
prominently discussed and should work to identify others worthy of
consideration. (What benefits does each alternative promise, other
than risk reduction?)
3. What is the effectiveness of each alternative? That is, how
much does it reduce the risks it is intended to reduce, and how is
the risk reduction distributed across relevant populations? (What
benefits does each provide, and how are they distributed?)
4. What are the costs of each alternative, and how are these
distributed across relevant populations?
Uncertainties In Knowledge About Risks and Benefits
Assessments of the risks and benefits of all available options, to
be complete, should address the following questions about their own
reliability:
1. What are the weaknesses of the available data? Information
needed to estimate the risks and benefits of an activity or substance
and the effects and costs of alternatives often does not exist. Some
times experts dispute the accuracy or reliability of the data that
are available. And often not enough is known to extrapolate confi-
dently from those data to estimates of risks (or benefits) for a whole
population.
2. What are the assumptions and models on which the estimates
are based when data are missing or uncertain or when methods of
estimation are in dispute? How much dispute exists among experts
about the choice of assumptions and models?
3. How sensitive are the estimates to changes in the assumptions
or models? That is, how much would the estimate change if it used
different plausible assumptions about exposures or incidences of harm
(or benefits) or different methods for converting available data into
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UNDERSTANDING HAZARDS AND RISKS
37
estimates? What are the boundaries or confidence limits within
which the correct risk (or benefit) estimate probably falls? What is
the basis for concluding that the correct estimate is not likely to lie
outside those bounds?
4. How sensitive is the decision to changes in the estimates?
That is, if, because of uncertainty, an estimate of risk or benefit were
wrong by a factor of 2, or 10, or 100, would the decision maker's
choice be different?
~ .
5. What other risk and risk control assessments have been made,
and why are they different from those now being offered?
Formation on Management
"Risk management" is a term used to describe processes sur-
rounding choices about risky alternatives. In common usage, assess-
ments of the risks and benefits of various options are seen as technical
activities that yield information for decision makers, whose decisions
are called risk management decisions (National Research Council,
1983a). tIf one accepts the distinction between risk assessment and
risk management (see the list of terms in Appendix E), communi-
cation about risks that involves nonexperts would generally be part
of risk management.]
1_ <1 1 · · ~
In addition to information about risks and
renews, Decision makers need answers to managerial questions such
as these:
1. Who is responsible for the decision? Who is responsible for
preventing, mitigating, or compensating for damage? Who is respon-
sible for generating and evaluating data? Who has oversight?
What issues have legal importance? Do the applicable laws
take benefits into consideration? Do they allow consideration of the
risks of alternatives? Do they require the analysis of economic en c!
social impacts of the activity in question or its alternatives?
3. What constrains the decision? What technical, physical, bi-
ological, or financial limits constrain some possible choices? What
are the limits of authority of the person or organization making the
decision? Are there time limits imposed on the decision process?
What difference could public opinion or political intervention make?
4. What resources are available for implementing the decision?
What personnel and financial resources are available to the decision
maker? To others involved in debating the decision?
2.
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IMPROVING RISK COMMUNICATION
Other Relevant Enowledge
In addition to items on the above lists, other considerations are
also important. Technological choices involve risks and benefits not
only to the life, health, and safety of individual humans but also to
nonhuman organisms, ecological balances, the structures of human
communities, political and religious values, and other things that
concern decision makers but that are not easily evaluated by the
quantitative approaches implied by the above lists. The assessment
of such risks and benefits is not standard practice in the field of
risk assessment. Such factors are commonly discussed, however,
in activities and documents described as "impact assessments" or
"technology assessments." These broadly conceived activities and
documents often address a wide range of the questions just outlined.
Summer
In sum, a weiZ-informed choice about activities that present haz-
ards and risks requires a wide range of knowledge. It depends on
understanding of the physical, chemical, and biological mechanisms
by which hazardous substances and activities cause harm; on knowI-
edge about exposures to hazards or, where knowledge is incomplete,
on analysis and modeling of exposures; on statistical expertise; on
knowledge of the economic, social, esthetic, ecological, and other
costs and benefits of various options; on understanding of the social
values reflected in differential reactions to the qualities of risks; on
knowledge of the constraints on and responsibilities of risk managers;
and on the ability to integrate these disparate kinds of knowledge,
data, and analysis. Needless to say, it is often impossible in practice
to gather at! this knowledge. Nevertheless, the more complete the
knowledge and the more quantitative answers are found, the better
informed the ultimate decision will be.
GAPS AND UNCERTAINTIES IN KNOWLEDGE
The above summary of needed knowledge clearly suggests that
decisions about risky activities and hazardous substances are fre-
quently made with incomplete information. In this section we elabo-
rate on some of the points just raised. We focus on risks, even though
there are major gaps and uncertainties in knowledge about benefits
as well, and we list several important ways that information about
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UNDERSTANDING HAZARDS AND RISKS
_-
_ _- s
39
"Then we've agreed that all the evidence isn't in, and that even if all
the evidence were in, it stid wouldn't be definitive."
FIGURE 2.1 SOURCE: Drawing by Richter; @1987 The New Yorker Maga-
z~ne, Inc.
the nature ant] magnitude of risk is often incomplete and uncertain
(see Figure 2.1~.
Identification of Hazards
It is sometimes difficult even to determine whether a hazard ex-
ists. For activities or substances whose hazards are delayed in onset
(such as possible causes of cancer or birth defects) and for substances
to which people are exposed in very small quantities, it is Circuit to
connect effects to causes. Analysts often use experiments with ani-
mals or bacteria to determine whether such activities or substances
are hazardous under controlled conditions, but not all potential haz-
ards are studied, even in the laboratory. A National Research Council
pane! reviewed the testing that had been done on a random sample
of 675 substances (National Research Council, 1984~. Within this
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IMPROVING RISK COMMUNICATION
group, 75 percent of the drugs and inert chemicals in drug formula-
tions had had some testing for acute toxicity and 62 percent had had
some testing for sub chronic effects. For pesticides and ingredients in
pesticide formulations, these values were 59 percent and 51 percent,
respectively. Testing for chronic, mutagenic, or reproductive and
developmental effects was less frequently done than testing for acute
and subchronic effects, and testing of all kinds was less frequently
done for substances on the Toxic Substance Control Act's list of
chemicals in commerce. The pane} concluded that toxicity studies
had not yet been done on the majority of the chemicals amounting
to tens of thousands now in industrial use in the United States.
Even when studies have been done with Tower organisms, it is
uncertain whether there is a human hazard. Substances that cause
cancer, mutations, or birth defects in some species of animals often
have no demonstrable effect on other species, and the reasons for
these differences are not yet understood. For instance, a review by
the Food and Drug Administration indicated that of 38 compounds
demonstrated or suspected to cause birth defects in humans, all
except one tested positive in at least one animal species and more
than 80 percent were positive in more than one species. Eighty-
five percent of the 38 compounds caused birth defects in mice, 80
percent in rats, 60 percent in rabbits, 45 percent in hamsters, and 30
percent in primates (National Research Council, 1986b). Thus some
substances that do not cause cancer or birth defects in test species
appear to have these harmful effects on humans. And the reverse
may also be true. Scientists may agree that positive results in an
animal test on a particular substance are strong evidence of a human
hazard, but there is always some uncertainty about that judgment.
Estimation of Exposure
Data are frequently inadequate on exposures to hazards. Many
hazardous substances are diffused in the air or in surface or under-
ground waterways and in the process undergo physical or chemical
changes that transform them into other substances that may be less
hazardous or that may be more so, although more dilute. Many
hazardous substances are transformed by biological processes before
they reach humans. And even in the human body, metabolic pro-
cesses can alter hazardous chemicals before they reach the organs
to which they present hazards, sometimes making them less toxic,
but sometimes making them more so (National Research Council,
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UNDERSTANDING HAZARDS AND RISKS
be
_ / ~-at_ ~
43
THE SCIENrlftC COMITY
IS DIVIDED.
SOI`tE 5~Y THIS STUFF IS
0P`NGEROU~OME SAY
11 ISN T.
_ ~-
~V
FIGURE 2.2 SOURCE: Drawing by Richter; 6)1988 The New Yorker Maga-
zine, Inc.
The uncertainties in these methods are legion, so several different
and even conflicting conclusions can often be defended by competent
scientists. It is difficult and sometimes proves impossible to reach
a consensual judgment about what the probabilities are, let alone
what to do about the attendant risks (see Figure 2.2~.
Identification of Synergistic Effects
Additional uncertainty in risk estimates exists because exposure
to one hazard can affect a person's sensitivity to another. For in-
stance, asbestos is estimated to be about 10 times as dangerous to
smokers as to nonsmokers (Bresiow et al., 1986~. This may occur
because chemical reactions between the substances yield products of
different toxicity or because one substance increases the availability
to the body of another one that would not have been toxic by it-
self (National Research Council, 1988a). In such ways, exposure to
one substance can potentiate the adverse effects of another or, less
commonly, decrease another substance's toxic effect. There is very
little knowledge, however, about how frequent or how strong such
synergistic or blocking effects are or about which combinations of
~ i,-
substances and activities are likely to exhibit the effects. The knowI-
edge that such effects exist, however, gives reason to consider almost
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IMPROVING RISK COMMUNICATION
all estimates of health risk based on studies of single hazardous sub-
stances as somewhat uncertain, even when they are based on the
most careful analysis possible.
Summed
In sum, any scientific risk estimate is likely to be based on in-
complete knowledge combined with assumptions, each of which is a
source of uncertainty that limits the accuracy that should be ascribed
to the estimate. Does the existence of multiple sources of uncertainty
mean that the final estimate is that much more uncertain, or can
the different uncertainties be expected to cancel each other out? The
problem of how best to interpret multiple uncertainties is one more
source of uncertainty and disagreement about risk estimates.
SCIENTIFIC JUDGMENT AND ERRORS IN JUDGMENT
What do analysts do when confronted with knowledge so full of
uncertainties? Scientists' training, which teaches them to accurately
represent certain types of uncertainties, comes into conflict with the
pressure to give succinct, unambiguous answers that can inform the
social and personal decisions nonexperts must make about risks. If
the experts remain silent or equivocal, choices will be made without
taking into account what they know. Once they begin to convey
what they know, however, experts must inevitably make judgments
about the meaning of available information and about the degree
to which uncertainty makes it less reliable. But because experts
rely on ordinary cognitive processes to make sense of the wealth of
data they have available, their judgments about the meaning and
conclusiveness of available information can suffer from some of the
same frailties that affect human cognition in general.
Inappropriate Reliance on I~ ted Data
Even statistically sophisticated individuals often have poor in-
tuitions about how many observations are necessary to support a
reliable conclusion about a research hypothesis (Tversky and Kah-
neman, 1971~. In particular, they tend to draw conclusions from
small samples that are only justified with much larger samples. Thus
they may be prone to conclude that a phenomenon such as a toxic
erect does not exist when in fact the data are so sparse that the only
appropriate conclusion is that the search for the phenomenon is in its
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UNDERSTANDING HAZARDS AND RISKS
45
early stages. They may also err in the opposite direction, sounding
an alarm on the basis of extremely limited prelirn~nary data. The
tendency for scientists to draw conclusions from "low-power" re-
search has been documented in fields from psychology (Cohen, 1962)
to toxicology (Page, 1981~. Low-power research uses- measurements
and methods that are unlikely to reveal small effects without very
large numbers of measurements. Where the tendency to premature
conclusion operates, expert judgment will err by underreporting or
overreporting effects, both hazardous and beneficial.
Tendency to :hnpose Order on Random Events
People who are seeking explanations for events, including ex-
perts working in their areas of expertise, have a tendency to see
meaning even when the events are random (Kahneman and Tversky,
1972~. For instance, stock market analysts develop elaborate theories
of market fluctuations, but their predictions rarely do better than
the market average (Dreman, 1979), and clinical psychologists see
patterns they expect to find even in randomly generated test data
(O'Leary et al., 1974~. In interpreting statistics relating the inci-
dence of cancer to occupational exposures to particular chemicals,
there is a temptation to interpret a correlation between exposure
to a particular chemical and the incidence of a particular cancer as
evidence of an effect. But some such evidence is to be expected even
in random data, if large numbers of chemicals and cancers are exam-
ined. Similarly, occasional "cancer clusters" are likely to be present
in large epidemiological studies even by chance. Replication on a new
sample is the best way to check the reliability of such relationships,
but new samples are often hard to find. Sometimes, conclusions are
reported and publicized as definite before they have been adequately
checked.
Such instances, including the interpretation of "unusual" cases,
are at heart issues of the proper conduct of scientific analysis. Al-
though recent attention on scientific misconduct may attach greater
significance to unusual cases than is actually warranted, it is nonethe-
less important to recognize the natural human tendency to find order
even when the evidence is tenuous and to recognize that when an-
alysts are strongly motivated to find particular results they may
overinterpret the evidence.
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IMPROVING RISK COMMUNICATION
Tendency to Fit Alribiguous Evidence into Predispositions
When faced with ambiguous or uncertain information, people
have a tendency to interpret it as confirming their preexisting beliefs;
with new data they tend to accept information that confirms their
beliefs but to question new information that conflicts with them
(Ross and Anderson, 1982~. Because of the high degree of ambiguity
in the data underlying risk assessments, this cognitive bias may act
to perpetuate erroneous early impressions about risks even as new
evidence makes them less tenable.
Tendency to Systematically Omit Components of Risk
In analyses of complex technological systems, certain features
are commonly omitted, possibly because they are absent from oper-
ating theories of how the technological systems work. In particular,
analysts are prone to overlook the ways human errors or deliberate
human interventions can affect technological systems; the ways dif-
ferent parts of the system interact; the ways human vigilance may
flag when automatic safety measures are introduced; and the pos-
sibility of "common-mode failures," problems that simultaneously
affect parts of the technological system that had been assumed to
be independent [for elaboration and citations of the evidence, see
Fischhoff et al. (1981a). Typically, people who severe not involved
in performing the analyses are unlikely to notice such omissions-in
fact, in a complex technical analysis, observers are likely to overlook
even major omissions in the analysis. Although most of these over-
sights tend to lead to underestimates of overall risk, this need not
always be the case.
Overconfidence in the Reliability of Analyses
Weather forecasters are remarkably accurate in judging their
own forecasts. When they predict a 70 percent chance of rain, there
is measurable precipitation just about 70 percent of the time. They
seem to be so successful because of the following characteristics of
their situation: (1) they make numerous forecasts of the same kind,
(2) extensive statistical data are available on the average probabil-
ity of the events they are estimating, (3) they receive computer-
generated predictions for specific periods prior to making their fore-
casts, (4) a readily verifiable criterion event allows for quick and
unambiguous knowledge of results, and (5) their profession admits
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UNDERSTANDING HAZARDS AND RISKS
47
its imprecision and the need for training (Fischhoff, 1982; Mur-
phy and Brown, 1983; Murphy and Winkler, 1984~. Most of these
conditions do not hold for professional risk assessors, however, and
the predictable result is overconfidence among experts. For instance,
civil engineers do not normally assess the likelihood that a completed
dam will fail, even though about 1 in 300 does so when first filled
with water (U.S. Committee on Government Operations, 1978~.2
Summary
These normal cognitive tendencies can lead expert risk analysts
to convey incorrect impressions of the nature and reliability of sci-
entific knowledge. Some of the tendencies predispose to premature
judgment that a risk is low or high. Several of them bias scien-
tific judgment in the direction of overconfidence about the certainty
of whatever currently seems to be known. Although the net effect
of these cognitive tendencies has not been determined, their exis-
tence justifies a certain amount of skepticism on the part of decision
makers, including individuals, about definitive claims made by risk
analysts.
INFLUENCES OF HUMAN VALUES ON
1[NOWLEDGE ABOUT RISE
Although it is useful conceptually to separate risk assessment
and risk control assessment from value judgment, there are many
respects in which it is not possible to accomplish the separation in
practice. Judgments made by scientists on which types of hazardous
consequences to study and by analysts on which ones to measure
are based in part on technical information what knowledge already
exists, what additional knowledge would be relevant to a decision at
hand, what the relative costs are of collecting different kinds of data,
and what kinds of information would be most useful for estimating
particular risks. But they are also based on value judgments about
which types of hazard are most serious and therefore most worthy of
being reduced. This section discusses two of the ways that human
values enter understanding of risks: through the choice of numbers
to summarize knowledge about the magnitude of risks and through
the weighting of different attributes of hazards.
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48
IMPROVING RISK COMMUNICATION
Choices of Numerical Measures for Risk
The need to quantify risks as an aid to decision making creates
special difficulties because the choice of which numerical measure
to use depends on values and not only on science. This fact is
evident even in a simple problem of risk measurement the choice
of a number to summarize information on fatalities. Different risk
analysts have used different summary statistics to represent the risk
of death from an activity or technology.3 Among the measures used
are the annual number of fatalities, deaths per person exposed or
per unit of time, reduction of life expectancy, and working days lost
as a result of reduced life expectancy. The choice of one measure
or another can make a technology Took either more or less risky.
For instance, in the period from 1950 to 1970, coal mines became
much less risky in terms of deaths from accidents per ton of coal,
but they became marginally riskier in terms of deaths from accidents
per employee (Crouch and Wilson, 1982~. This is because with
increasing mechanization fewer workers were required to produce the
same amount of coal. So although there were fewer deaths per year
in the industry, the risk to an individual miner actually increased
during this period. Which measure is more appropriate for decisions
depends on one's point of view. As some observers have argued,
"From a national point of view, given that a certain amount of
coal has to be obtained, deaths per million tons of coal is the more
appropriate measure of risk, whereas from a labor leader's point of
view, deaths per thousand persons employed may be more relevant"
(Crouch and Wilson, 1982:13~.
Each way of summarizing deaths embodies its own set of values.
For example, "reduction in life expectancy" treats deaths of young
people as more important than deaths of older people, who have less
life expectancy to lose. Simply counting fatalities treats deaths of
the old and young as equivalent; it also treats as equivalent deaths
that come immediately after mishaps and deaths that follow painful
and debilitating disease or Tong periods during which many who will
not suffer disease live in daily fear of that outcome. Using "number
of deaths" as the summary indicator of risk implies that it is equally
important to prevent deaths of people who engage in an activity
by choice and deaths of those who bear its effects unwillingly. It
also implies that it is equally important to protect people who have
been benefiting from a risky activity or technology and those who
get no benefit from it. One can easily imagine a range of arguments
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UNDERSTANDING HAZARDS AND RISKS
49
to justify different kinds of unequal weightings for different kinds
of deaths, but to arrive at any selection requires a value judgment
concerning which deaths one considers most undesirable. To treat
the deaths as equal also involves a value judgment.
There are additional value choices involved in calculations based
on fatalities. A particularly controversial choice concerns whether
to "discount" lives, that is, whether to give deaths far into the
future less weight than present deaths. This approach to valuation
is sometimes advocated on the ground that people typically prefer a
given amount of any particular good in the present to the same value
in the future if they invested the cost of the good, they could expect
to have increased purchasing power and thus to be able to purchase
more of it in the future than in the present. Although one cannot
"invest" human life in the same way society can invest the resources
used to save or prolong lives. From an individual's point of view,
one arguably loses less by dying at an old age than when younger,
so people may be less willing to work to avoid probable deaths the
farther they are in the future.
Discounting is controversial partly because it is used to put a
monetary value on human life. Some measure, whether based on
probable future earnings or consumption or on willingness to pay
to reduce the probability of fatality, is selected to put a price on
what for many has intrinsic moral or even religious value and each
of these measures embodies controversial assumptions about what
is worthwhile about life. In addition, choosing a positive discount
rate- one that treats future lives as worth less than present lives-
suggests that society cares less about its children's generation than
its own, a controversial assumption to say the least. But deciding
not to discount lives also involves a judgment about the future, and
~: ~: ~1 ~_1__ _ 1_ ~_ _1 · { ~1 ~
.
an '~ '~ "~o ~ va~ue-lauen cnolce ~^ec~nauser and Shephard, 1981).
Values also enter into scientists' choices about how to character-
ize the uncertainty in their information. It is traditional among civil
engineers, public health professionals, and others to take account of
uncertainty by being "conservative" in stating risk estimates. This
means that they leave a margin for error that will protect the public
if the actual risk turns out to be greater than the best currently
available estimate. But it has sometimes been argued that risk ana-
lysts should instead present their best available estimate to decision
makers, along with an explicit characterization of its uncertainty,
and allow the decision makers to decide explicitly how much margin
of safety to allow. The dispute is highly controversial because many
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50
IMPROVING RISK COMMUNICATION
~1
it.
Em, SO I,_
" HEY, ~ THO~HT WE WEE No Girl THE ME Did..."
FIGURE 2.3 SOURCE: National Wildlifc Magaxinc, August-September, 1984.
Copyright Hi) 1984 Mark Taylor. Reprinted with permission of Mark Taylor.
believe that in practice the latter approach will provide a narrower
margin of safety. The central point here is that either way of repre-
senting uncertainty embodies a value choice about the best way to
protect public health and safety.
These few examples show how human values can enter into even
apparently technical decisions in risk analysis, such as about the
choice of a number to summarize a body of data. It is easy therefore
to see how choices that are justified by appeal to data from a risk
analysis can sometimes be questioned by appealing to the very same
data (see Figure 2.3~.
Values and the Attributes of Hazards
We have noted that decision makers do not choose among risks
but among alternatives, each with many attributes, only some of
which concern risk. Similarly, each hazard and, for that mat-
ter, each benefit that a decision alternative presents has many
attributes. These attributes are important to nonexperts for the
purpose of making decisions.
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UNDERSTANDING HAZARDS AND RISKS
51
Qualitative aspects of hazards are relevant to decisions in various
ways. In different decision contexts it may be necessary to consider
comparisons and trade-offs such as the following: Is a risk of cancer
worse than a risk of heart disease? Is an accidental death of a
person at age 30 more to be avoided than a death by emphysema
at age 70? Is an industrial hazard more acceptable if it is borne
by workers partly compensated by their pay than if it is borne by
nonworking neighbors of the industrial plant? Are the deaths of 50
passengers in separate automobile accidents equivalent to the deaths
of 50 passengers in one airplane crash? Is a hazard that faces the
unborn worse than a similar hazard that we face ourselves? Is a
large hazard with a Tow probability equally undesirable as a small
hazard with a high probability when the estimated risks are equal?
The difficult questions multiply when hazards other than to human
health and safety are considered. Technological choices sometimes
involve weighing the value of a river vista, a small-town style of living,
a holy place, or the survival of an endangered species, in addition to
dangers to human health, against probable economic benefits. Such
choices are ultimately matters of values and interests that cannot be
resolved merely by determining what the risks and benefits are.
A growing body of knowledge on what is usually called "risk
perception" helps illuminate the values involved in the evaluation of
different qualities of hazards.4 In studies of risk perception individ-
uals are given the names of technologies, activities, or substances
and asked to consider the risks each one presents and to rate them,
in comparison with either a standard reference or the other items
on the list. The responses are then analyzed, taking into account
attributes of the hazards and benefits each technology, activity, or
substance presents (Table 2.1 lists several such attributes). Analysis
consistently shows that people's ratings are a function not only of av-
erage annual fatalities according to the best available estimates, but
also of the attributes of the hazards and benefits associated with a
technology, activity, or substance (Fischhoffet al., 1978; Gould et al.,
1988; Otway and von WinterfelUt, 1982; SIovic et al., 1979, 19803. In
particular, the studies show that certain attributes of hazards, such
as the potential to harm large numbers of people at once, personal
uncontrollability, dreaded effects, and perceived involuntariness of
exposure, among others (see Table 2.1), make those hazards more
serious to the public than hazards that lack those attributes. Also,
choices that provide different types of benefit, such as money, secu-
rity, and pleasure, are valued differently from each other (Gould et
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52
IMPROVING RISK COMMUNICATION
al., 1988~. The fact that hazards differ dra~natically in their quaTita-
tive aspects helps explain why certain technologies or activities, such
as nuclear power, evoke much more serious public opposition than
others, such as motorcycle riding, that cause many more fatalities.
An important implication of such findings is that those quanti-
tative risk analyses that convert all types of human health hazard
to a single metric carry an implicit value-based assumption that all
deaths or shortenings of life are equivalent in terms of the importance
of avoiding them. The risk perception research shows not only that
the equating of risks with different attributes is value laden, but also
that the values adopted by this practice differ from those held by
most people. For most people, deaths and injuries are not equal-
some kinds or circumstances of harm are more to be avoided than
others. One need not conclude that quantitative risk analysis should
weight the risks to conform to majority values. But the research does
suggest that it is presumptuous for technical experts to act as if they
know, without careful thought and analysis, the proper weights to
use to equate one type of hazard with another. When lay and expert
values differ, reducing different kinds of hazard to a common metric
(such as number of fatalities per year) and presenting comparisons
only on that metric have great potential to produce misunderstanding
and; conflict and to engender mistrust of expertise.
IMPLICATIONS FOR RISE COMMUNICATION
We have shown in this chapter that different experts are likely to
see technological choices in different, sometimes contradictory, ways
even when the information is not at issue. Incomplete and uncer-
tain knowledge leaves considerable room for scientific disagreement.
Judgments about the same evidence can vary, and both judgments
and the underlying analyses can be influenced by the values held by
researchers. Since scientists and the people who convert scientific
information into risk messages do not all share common values, it
is reasonable to expect risk messages to conflict with each other.
Even in the best of circumstances for communication, conflicting
risk messages would create confusion in the minds of nonexperts who
must rely on them to inform their choices. But as the next chapter
shows, the circumstances are not the best. The social conflict that
surrounds modern technological choices is characterized by anxiety
and mistrust and by clashes of vested interests and values, conditions
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UNDERSTANDING HAZARDS AND RISKS
53
that create formidable tasks for those who would improve decision
making through risk communication.
NOTES
1. One technical definition of risk is that risk is the product of a measure
of the size of the hazard and its probability of occurrence. Regardless of how
numerical estimates are made, the essence of the distinction between hazard
and risk is that "risks takes probability explicitly into account.
2. This discussion is drawn from Fischhoff et al. (1981a). More extensive
discussions of expert overconfidence with additional examples can be found
there and in Lichtenstein et al. (1982~.
3. This discussion is drawn from Fischhoff et al. (1984:125-126), where
further citations can be found.
4. The term "risk perceptions is put in quotation marks because, as the
discussion shows, this body of research is more accurately described as the
study of human values regarding attributes of hazards (and benefits).
Representative terms from entire chapter:
risk communication
