Review of Recommendations for Probabilistic Seismic Hazard Analysis Guidance on Uncertainty and Use of Experts (1997) / Chapter Skim
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PROCESS AND DOCUMENTATION FOR A PROBABILISTIC SEISMIC HAZARD ANALYSIS
PROCESS AND DOCUMENTATION FOR A PROBABILISTIC SEISMIC HAZARD ANALYSIS
Pages 15-30
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From page 15... ...
SSHAC lays out two basic principles underlying the PSHA process and its results: Regardless of the scale of a PSHA study, the goal (as stated by SSHAC) is "to represent the center, the body, and the range of technical interpretations that the larger technical community would have if they were to conduct the study.
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From page 16... ...
These principles underlie the primary recommendations of the SSHAC report that deal with the PSHA process. LEVEL OF EFFORT IN A PSHA SSHAC recognizes that a PSHA can be carried out at different levels of effort and emphasizes that the effort expended should match the importance of the facility, the degree of controversy, uncertainty, and complexity associated with the relevant scientific issues, and external decision factors, such as regulatory concerns and the resources available.
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From page 17... ...
Reliance on a single entity (TI) to characterize the input of the whole technical community may be a very efficient mode of operation, but additional assurance is needed to provide confidence that the results are a reasonable representation of the community's views.
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From page 18... ...
Issue Degree Decision Factors Study Level Non-controversial; and/or TI evaluates/weights models insignificant to hazard based on literature review and experience; estimates community distribution B · Regulatory concern 2 Significant uncertainty and · Resources available TI interacts with proponents diversity; controversial; and · Public perception & resource experts to identify complex issues and interpretations; estimates community distribution Highly contentious; significant TI brings together proponents to hazard; and highly complex & resource experts for debate and interaction; n focuses debate and evaluates alternative interpretations; estimates community distribution 1~1 organizes panel of experts to interpret and evaluate; focuses discussions; avoids inappropriate behavior on part of evaluators; draws picture of evaluators' estimate of the community's composite distribution; has ultimate responsibility for project that have relied on formally elicited expert judgment have strived to get experts to think in this manner. The hope was that the experts' composite view also represented the composite view of the technical community as a whole.
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From page 19... ...
SSHAC also strongly recommends a formal nomination process based on consulting the literature and asking technical societies, government orgaruzations, and knowledgeable individuals to submit the names of potential experts. Whatever the issue or structure of elicitation, the pane]
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From page 20... ...
The TFT process is centered on the precept of thorough and well-documented expert interaction as the principal mechanism for integration. As SSHAC acknowledges, a major stimulus for its charge was the need to resolve the differences in hazard estimates between the Lawrence Livermore National Laboratory and the Electric Power Research Institute studies.
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From page 21... ...
Appendix ~ of the SSHAC report spells out the background, evolution, and details of the TFT process as developed by SSHAC. Appendix ~ must be read carefully; readers may need to consult additional references in order to fully understand some of the issues discussed, such as the weighting of individual expert inputs.
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From page 22... ...
At each step of the elicitation process, the TFT strives for complete understanding by each expert of all technical issues. The goal is that all experts are "on the same page." The results of two ground motion workshops conducted by SSHAC and documented in Appendixes A and B of its report indicate that investment in the TFT process bore substantial results.
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From page 23... ...
Although this interpretation of the TFT mode} is not given in the SSHAC report, the fact that the community distribution is defined as a weighted sum of the expert distributions is equivalent to saying that each expert is correct with a probability equal to his/her assigned weight. At the end of Appendix I, the two approaches are compared numerically and shown to produce very different results.
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From page 24... ...
This pane! could think of no straightforward procedure to do so (one would need to consider the expert distribution estimates as random processes given the true community distribution function, with serious practical and conceptual implications)
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4. When combining expert opinions on distribution functions, the correct Bayesian approach requires the use of a random process formalism, unless the problem can be reduced to a discrete one through appropriate parameter~zation.
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From page 26... ...
As shown in the second SSHAC ground motion workshop, this interactive process narrowed the range of estimates as the experts increased their knowledge and understanding of issues and methods. One goal of a well-executed TFl process is that all participating experts are better able to make informed independent judgments.
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From page 27... ...
The late-stage review is closer to the traditional academic review in that it occurs near the end of a project. SSHAC strongly recommends participatory peer review on the grounds that a late-stage review can be risky, especially with regard to the process aspects of a PSHA study.
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From page 28... ...
The pane] recommends that specialized computer programs needed to implement the SSHAC procedures be readily accessible to any group that wants to engage in seismic hazard evaluation as part of a research program or business venture.
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From page 29... ...
Process and Documentation for a PSHA 29 The pane} concludes that the discussion of the documentation process in Chapter 7 of the SSHAC report provides thorough and useful guidance for numerous other applications in addition to seismic hazard assessment. Documentation is not one of the more glamorous aspects of the scientific enterprise, but it is essential to the fuN realization of the benefits of the large investment in data acquisition, analysis, and interpretation that are characteristic of large projects.
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