Indexes of severity: conceptual development

A discussion of severity index development is presented in relation to conceptual issues in index definition, analytic issues in index formulation and validation issues in index application. The CHOP index is discussed along with six severity indexes described in an earlier paper dealing with underlying concepts to illustrate the material presented. Replies are provided to specific questions raised in an accompanying paper discussing the Injury Severity Score. This conceptual material is presented to provide a foundation for severity index development, to suggest criteria to be used in their formulation and testing, and to identify analyses that can lead to the successful selection and application of an index for a defined purpose.