INDEXES OF FOOD AND NUTRIENT INTAKES AS PREDICTORS OF SERUM CONCENTRATIONS OF NUTRIENTS - THE PROBLEM OF INADEQUATE DISCRIMINANT VALIDITY

Nutrient indexes derived from food-frequency questionnaires have gener ally been regarded as acceptably valid for epidemiologic purposes. Eva luations of these indexes, however, have considered only their converg ent validity. We suggest that discriminant validity, or the ability to distinguish among exposures to different nutrients, is also important . Using baseline data from a large clinical trial, we tested the discr iminant validity of indexes of intake of vitamin E, alpha-carotene, an d beta-carotene. Our results suggest that the vitamin E index possesse s neither convergent nor discriminant validity, the alpha-carotene ind ex adequate convergent and discriminant validity, and the beta-caroten e index adequate convergent but no discriminant validity.