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FIGURE 3.2 Ranking the risk of bioagents—the principal product of the Biological Threat Risk Assessment (BTRA) of 2006. In this figure, biological agents versus normalized risk, a sample display is based on fictitious data that represents only the general appearance of a key BTRA result. One of the vertical bars in this sample display represents anthrax; the dot shows the mean expected fatalities, and the horizontal bars show the 5th and 95th percentiles. However, as is done in the analyses included in DHS (2006), the vertical scale has been normalized so that the sum of the mean risks over all agents is 1. The committee does not know the normalization constant applied by BTRA and so cannot recover the actual expected risks.

1994 U.S. Nuclear Regulatory Commission reports and later applications depend.

The principal product of the BTRA of 2006 was a ranking of the risk posed by bioagent use based on calculated probabilities of expected fatalities. DHS chose to assess threat by ranking bioagents because government stakeholders had advised DHS that they “expected the primary assessments to be in the form of risk-prioritized groups of biological threat agents” (DHS, 2006, Ch. 1). Although a terrorist’s choice of agent is just one step in a sequence of events leading to a potential attack, for practical purposes the BTRA of 2006 evaluates each agent separately. A probability is computed for each scenario involving that agent. Risk is then calculated as the product of these probabilities and the associated consequences. The overall risk associated with each agent is the integrated risk distribution over all possible scenarios involving that agent.

The product of the analysis by the BTRA of 2006 is displayed in a figure (such as Figure 3.2) that shows, for each agent, a normalization (whose normalization constant is not defined) of three estimated parameters of the distribution of consequences of agent attack in terms of expected fatalities:1

  • The 5th percentile,

  • The expected value (or mean), and

  • The 95th percentile.

For each agent, the estimate of the 5th percentile and of the 95th percentile of expected fatalities is displayed as a tick mark on a vertical line on a logarithmic ordinate scale of (normalized) consequences. The mean of expected fatalities is displayed as a dot. A typical display shows 28 parallel vertical lines, one for each agent. The specific numbers and rankings of agents by risk are functions of the assumptions underlying each of the many steps in the model’s execution.

Before results are presented in DHS (2006), a normalizing constant is computed by multiplying, for each agent, the conditional expected consequence of the agent’s use by the probability of its use, and then summing over all the agents. All statistics are divided by this constant to force the normalized means to sum to 1. This critical normalization constant is not displayed in the DHS (2006) report, so no absolute (versus relative) consequence can be recovered from the analysis presented there. Therefore, the normalization method cannot be verified by the committee. The normalization step is a curious one, in that it damages the results irreparably for purposes of decision making about, for instance, risk management. The committee conjectures that the normalization may reflect a well-intentioned but nonetheless an unfortunate effort to mitigate the stark nature of the estimated risks reported.

DHS (2006) also contains some qualitative analysis distinguishing between most-, less-, and least-“worrisome” bioagents. As for the quantitative analyses, consequences include only immediate numbers of expected fatalities. Future assessments have been promised with estimated casualties


The analyses presented in DHS (2006) are based entirely on estimated fatalities. However, DHS has conducted assessments based on illnesses and direct economic consequences as well.

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