(McDonald, 1996) and an awareness that the risks in the BR range from theoretical concerns (e.g., virus-induced lymphomas and leukemia) to practical issues (e.g., nutrition, motion sickness, and bone and muscle loss). In addition, some risks are specific (e.g., renal stone formation), whereas others are general (e.g., ambulatory care).
The committee believes that risk assessment should primarily be evidence-based wherever possible, and where evidence does not exist, research should be directed to acquiring the evidence needed (see example on bone fracture risk associated with prolonged exposure to microgravity, Appendix D). Rating the quality of each published source of information should be a component of the BR. Quality ratings should follow commonly used criteria such as those of the Cochrane Collaboration (Starr and Chalmers, 2005) or the Agency for Healthcare Research and Quality (AHRQ, 2002) National Guideline Clearinghouse, recognizing that quality rating methods based only on the published literature will have to be modified to accommodate other sources such as conclusions from workshops and planning meetings. For risks that have more than one information source, the quality of the most robust source should be noted.
The committee recommends that NASA determine, and incorporate into the BR, measures of the quality of the evidence that forms the basis for defining risks and the assessments associated with each risk.
Currently, the status of risks is represented in the BR as the aggregation of the importance of the risk (probability of occurrence × severity of consequences) combined with the effectiveness of the current state of counter-measures. In this model, a “severe” risk to the crew—such as loss of breathable atmosphere—would be represented as being minimal if the state of life support technology readiness or life support capability readiness to mitigate the risk was high. The committee notes that it is important to explicitly and separately assess and represent the importance of each risk and the state of the mitigation strategy or countermeasures that address the risk.
Disaggregation of mitigations is important for several reasons. First, it is important because aggregated risk values obscure the risk itself and can lead to false confidence or concern, depending on the status of counter-