Indexes of severity: underlying concepts.

Six severity indexes proposed in the health services research literature are shown to be special cases of a class of ordinal ranking functions called additive value functions. Deficiencies manifest in each of the severity indexes are discussed in relation to the properties that additive value functions must satisfy. In that severity indexes are a subset of health status indexes, the properties that imply and are implied by additive value formulations are relevant to the larger class of indexes as well.