Tests or measures in clinical medicine or the social sciences can be used for three purposes: discriminating between subjects, predicting either prognosis or the results of some other test, and evaluating change over time. The choices made at each stage of constructing a quality of life index will differ depending on the purpose of the instrument. We explore the implications of index purpose for each stage of instrument development: selection of the item pool, item scaling, item reduction, determination of reliability, of validity, and of responsiveness. At many of these stages, not only are the requirements for discriminative, predictive, and evaluative instruments not complementary, they are actually competing. Attention to instrument purpose will clarify the choices both for those developing quality of life measures and for those selecting an appropriate instrument for clinical studies.
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