Impact of differential item functioning on age and gender differences in functional disability.

Objectives: Estimates of group differences in functional disability may be biased if items exhibit differential item functioning (DIF). For a given item, DIF exists if persons in different groups do not have the same probability of responding, given their level of disability. This study examines the extent to which DIF affects estimates of age and gender group differences in disability severity among adults with some functional disability.

Methods: Data came from the 1994/1995 National Health Interview Survey Disability Supplement. Analyses focused on 5,750 adult respondents who received help or supervision with at least one of 11 activities of daily living/instrumental activities of daily living tasks. We estimated gender and age group (18-39, 40-69, and 70+) differences in disability, using multiple-indicator/multiple-cause models, which treat functional disability as a latent trait.

Results: Nine items manifested significant DIF by age or gender; DIF was especially large for "shopping" and "money management." Without adjusting for DIF, middle-aged persons were less disabled than elderly men, and women were less disabled than men among nonelderly persons. After adjusting for DIF, middle-aged persons did not differ from elderly persons, and gender differences within age groups were not significant.

Discussion: Comparisons of disability across sociodemographic groups need to take DIF into account. Future research should examine the causes of DIF and develop alternative question wordings that reduce DIF effects.