Summary Measurement: The Role of Categories or Domains

The focus of this chapter is on how the constant stream of information that a person experiences every day is summarized into units that are used to communicate or provide the basis for quantification. Two methods are described: those based on the person using cognitive processes to form categories of information and those that involve the investigator using statistical procedures to create domains. In addition, I describe how the content (modular or nonmodular) of these summary statements can vary and what it implies about the summary measure formed. Finally, I illustrate how differences in the content of a domain and the quality-of-life assessment used (generic or disease specific) contribute to the predictive ability of a quantitative assessment.