components must be quantified for the model or parameter under consideration. The panel has more trouble with this element than any other in the SSHAC report.

Recognition of the two kinds of uncertainty is useful initially when eliciting and combining expert inputs. Experts need to be aware of the sources of uncertainties (e.g., limitations of available data) so that they can make informed assessments of the validity of alternative hypotheses, the accuracy of alternative models, and the value of data and then transmit those uncertainties to the TFI. However, as detailed in Chapter 3 of this report, the panel believes that the statistical analysis and uncertainty separation procedures recommended by SSHAC may in some cases be more sophisticated than is warranted by the data or the purposes for which the results are to be used.

During the planning of a PSHA, a detailed analysis of uncertainty would be helpful but typically is not available. It may be sufficient for planning purposes to conduct limited sensitivity analyses, using bounding hypotheses, and to consider the level of effort that would be required to reduce the associated uncertainty.

In addition, the value of an epistemic/aleatory separation to the ultimate user of a PSHA is doubtful. In particular, it is not clear that such a separation would be more helpful than the display of expert-to-expert variability of a mean hazard at the time of an analysis, with an explanation of the source of the differences.

The panel also notes that the SSHAC report's discussions and recommendations on uncertainty and the use of experts are quite independent of PSHA and can be applied to other types of risk analysis. The panel believes that the SSHAC report makes a solid contribution to the methodology of hazard analysis, especially in the use of expert opinion.

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