measure or technology development. Second, the functioning of many parts of the space flight system is closely linked to the functioning of other parts of the system, and approaches that are used to mitigate one risk may have a positive or negative impact on other risks or mitigations. Finally, as the space flight system evolves, changes in systems may have a positive or negative impact on the mitigations. Failure to track risk separately from risk mitigation could well lead to failure to focus on the inherent relationships among risks and mitigations. For example, the risk of high “g’s” for a fighter pilot can be mitigated adequately by a “g-suit” system that is inflated by air pressure, and disaggregation of the mitigation from the risk helps ensure that system designers are aware that any changes that may affect the ability of the system to inflate the g-suit could affect the mitigation of the risk of g-induced incapacity. The committee observes that this disaggregation will also help maintain a clear understanding that the notion of “retiring risks” due to the availability of effective countermeasures is seldom an accurate depiction of the state of operational readiness, since the underlying health and safety issues (e.g., loss of breathable atmosphere) remain a concern and are not “retired” by the existence of a life support system.

Recommendation 3.2

The committee recommends that NASA structure the BR to represent separately the severity and likelihood of each risk and the state of the mitigation strategy or countermeasures associated with each risk.

Expressing Uncertainty

The determination of risk always involves an element of uncertainty. To fully communicate the likelihood of occurrence of an event, it is necessary to communicate the extent of uncertainty in the assessment. The uncertainty associated with a risk may be represented by objective measures such as statistical confidence intervals and sensitivity analyses (NCRP, 1996, 1997; Warren-Hicks and Moore, 1998; Grogan et al., 2000; NCI– CDC, 2003) or by less objective but potentially useful techniques such as approximate reasoning (Hayes et al., 1979) or possibility theory using fuzzy sets (Zadeh, 1978). It is also possible to state uncertainty using narrative descriptions of the risk, such as expert opinion obtained in focus group settings (Cacuci, 1988; Lash and Silliman, 2000). The current printed and on-line versions of the BR do not include any expression of uncer-



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