Differential item functioning in the Cambridge Mental Disorders in the Elderly (CAMDEX) Depression Scale across middle age and late life.

A long-standing and critical problem in the study of aging and depression is the comparability of measurement across age groups. While psychological measures of depression typically show increased incidence of symptoms with increasing age, rates of depression diagnosis do not show the same age trend. This analysis presents tests of differential item functioning on the depression section of the CAMDEX interview schedule, using factor analysis-derived affective and somatic subscales (McGue & Christensen, 1997). Results for the affective subscale show significant differences in item functioning in the majority of the affective items as a function of age (items "Happy Life," "Lonely," "Nervous" "Worthless," and "Future": χ6(2) = [30.193, 255.971] across items, all p < .0001). Analyses for the somatic subscale show differential item functioning is limited to a single item relating to coping (χ6(2) = 180.754, p < .0001). These results indicate that differences in depression symptoms across age groups are not entirely consistent with a unidimensional depression trait, and that the measurement structure of depression varies over the life span.