Providing a structural framework within which information and judgments are used and combined does not of itself improve the quality of currently available information (although further research to generate new data might be guided by such a framework). Nor does it reduce the range of opinion likely to be expressed in predictions, judgments, or estimates (except as issues are more precisely defined).
The committee believes that the system it proposes is the most appropriate for the desired purpose and has implemented it with the best available data and estimates. The committee believes that the system would improve the decision-making process by making it more accessible to evaluation and reconstruction by other decision makers and by facilitating examination of the effect of alternative assumptions or estimates. However, some cautions and comments are needed to prevent misinterpretation of the power and precision of the method.
To identify the components on which quantitative information is desirable (though not necessarily available), the system (more than others) exposes areas of ignorance and uncertainty in which expert judgment, by necessity, must be used. The proposed approach uses equations to define the way in which information or estimates are combined (something not always specified in other approaches); this does not imply that the components or the results have the accuracy sometimes associated with formal mathematical calculations. The results are simply the consequence of combining both factual and uncertain quantities, both objective and subjective elements, that are an inescapable part of reaching conclusions about the preferred investments in new vaccines.
A quantitative structured model facilitates examination of the effect of uncertainty (in data and estimates) in a way that intuitive integration of such components does not. This is expressed in the sensitivity analyses reported in the study.
All processes in which diseases are ranked by importance involve value judgments on disease conditions; incorporation of value judgments may be explicit, implicit, or unrecognized. These judgments are more subjective than those of a scientific nature. Providing a specific point at which the required value judgments are described and incorporated is one means of isolating these differences of opinion (which are often incorporated into decision making in an ill-defined way) and determining if they affect the ultimate priorities.
The committee considered these problems, resolved differences of opinion, and sought agreement on the approach it would follow in this complex area. When information was incomplete or quantitative prediction was complicated by many unresolved issues, it chose what it believed was the most rational approach to selecting priorities, recognizing that exact data on all components required by the system would not be available before decisions had to be made. Because of the uncertain data and estimates used in the calculation of health benefits and costs, the final numerical rankings are useful as they relate to each other rather than because of their absolute precision. That is, the system facilitates comparison of vaccine projects in a way that is open to revision if different estimates or assumptions seem appropriate and as new data become available.