United States Nuclear Regulatory Commission - Protecting People and the Environment

Approaches to Acceptable Risk: A Critical Guide (NUREG/CR-1614)

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Publication Information

Manuscript Completed: September 1980
Date Published: December 1980

Prepared By:
Baruch Fischhoff
Sarah Lichtenstein
Paul Slovic
Ralph Keeney 1
Stephen Derby 2

Decision Research for
Oak Ridge National Laboratory
Oak Ridge, Tennessee 37830
Under ORNL Subcontract No. 7656

1 Woodward-Clyde Consultants
Embarcadero Center
San Francisco, California 94111

2 Stanford University
Stanford, California 94305

NRC FIN No. B0424

Prepared for:
U.S. Nuclear Regulatory Commission
Washington DC 20555-0001
NRC Interagency Agreement 40-550-75

Availability Notice


Acceptable-risk decisions are an essential step in the management of technological hazards. In many situations, they constitute the weak (or missing) link in the management process. The absence of an adequate decision-making, methodology often produce s indecision, inconsistency, and dissatisfaction. The result is neither good for hazard management nor good for society.

This report offers a critical analysis of the viability of various approaches as guides to acceptable-risk decisions. It does so by:

(1) Defining acceptable-risk decisions and examining some frequently proposed, but inappropriate, solutions.

(2) Characterizing the essential features of acceptable-risk problems that make their resolution so difficult. These are: uncertainty about how specific decision problems are to be defined, difficulties in ascertaining crucial facts, the problematic nature of the value issues that arise, the vagaries of human behavior that render, responses to hazards unpredictable, and inability to assess the adequacy of decision-making processes and the degree to which their conclusions are to be trusted.

(3) Creating a taxonomy of decision-making methods, identified by how they attempt to address the features of acceptable-risk problems listed below. The major categories discussed here are:

  • Professional judgment: allowing technical experts to devise solutions;
  • Bootstrapping: searching for historical precedents that embody guides to future decisions; and
  • Formal analysis: -theory-based procedures for modeling problems and calculating the best decision.

(4) Specifying the objectives that an approach should satisfy in order to guide social policy. These are: comprehensiveness, logical soundness, practicality, openness to evaluation, political acceptability, institutional compatibility, and conduciveness to learning.

(5) Rating the success of the approaches in meeting these objectives. Namely: How well does each approach satisfy each objective? How satisfactory are the approaches relative to one another? How might one choose the most adequate approach for different decision problems?


The following conclusions emerge from our analysis:

(1) Acceptable-risk problems are decision problems, that is, they require a choice between alternatives. That choice depends upon the alternatives, values, and beliefs that are considered. As a result, there is no single all-purpose number that expresses "acceptable risk" for a society.

(2) Values and uncertainties are an integral part of every acceptable-risk problem. As a result, there are no value-free processes for choosing between risky alternatives. The search for an "objective method" is doomed to failure and may blind the searchers to the value-laden assumptions they are making.

(3) None of the approaches considered here offers an unfailing guide to selecting the most acceptable alternative. Each gives special attention to some features of acceptable-risk problems,, while ignoring others. As a result, not only does each approach fail to give a definitive answer, but it is predisposed to representing Particular interests and recommending particular solutions. Hence, choice of a method is a political decision with a distinct message about who should rule and what should matter.

(4) Acceptable-risk debates are greatly clarified when the participants are committed to separating issues of fact from issues of value. Yet, however sincere these attempts, a clear-cut separation is often impossible. Beliefs about the facts of the matter shape our values; in turn, those values shape the facts we search for and how we interpret what we find.

(5) The controlling factor in many acceptable-risk decisions is how the problem is defined (i.e., which options and consequences are considered, what kinds of uncertainty are acknowledged, and how key terms are operationalized). As a result, definitional disputes underlie some of the most rancorous political debates.

(6) Values, like beliefs, are acquired through experience and contemplation. Acceptable-risk problems raise many complex, novel, and subtle value issues, for which we may not have well-articulated preferences. In such situations, the values we express may be greatly influenced by transient factors, including subtle aspects of how the question is posed.

(7) Even the most knowledgeable experts may have an incomplete understanding of new and intricate hazards. Indeed, some limits on breadth of perspective may be a concomitant of acquiring a particular disciplinary or world outlook. In such cases, non-experts may possess important supplementary information or viewpoints on hazards and their consequences.


No one solution to acceptable-risk problems is now available, nor is it likely that a single solution will ever be found. Nonetheless, the following recommendations, addressed to regulators, citizens, legislators, and professionals, should, if implemented, enhance society's ability to make decisions.

(1) Explicitly recognize the complexities of acceptable-risk problems. The value judgments and uncertainties encountered in specific decision problems should be acknowledged. More generally, we should realize that there are no easy solutions and not expect them from society's decision makers.

(2) Acknowledge the limits of currently available methods and expertise. Since we do not know how to get the right answers to these questions, we should concentrate on avoiding the mistakes to which various disciplines and people are attuned. The result would be a multi-method, multi-perspective approach to decision making that emphasized comprehensiveness.

(3) Improve the use of the present approaches. Develop guidelines for their conduct and review. Make their scope and presentation sensitive to all aspects of the problem and to the desires of -as many shareholders as possible. Analyses should be repeated in order to incorporate the insights they engender and the critiques they provoke.

(4) Make the decision-making process consistent with existing democratic institutions. The public and its representatives should be constructively involved in the process in order to legitimate its conclusions, facilitate their implementation, and increase the public's understanding of hazard issues.

(5) Strengthen non-governmental social mechanisms that regulate hazards. Decisions reached in the marketplace and political arena provide important guidance to most approaches. Their functioning can be improved by various measures including reform of the product liability system and increased communication of risk information to workers and consumers.

(6) Clarify government involvement. Legislation should offer clear, feasible, predictable mandates for regulatory agencies. The management of different hazards should be coordinated so as to build a legacy of dependable precedents and encourage consistent decisions.

If followed, these recommendations will- help create the conditions for society to learn from its day-to-day experience in making acceptable-risk decisions and living with their consequences. A final chapter of this report provides an agenda for scientific, research to complement this learning by doing.

Page Last Reviewed/Updated Thursday, August 08, 2013