Item-level psychometrics of the ADL instrument of the Korean National Survey on persons with physical disabilities

Purpose: The aim of this study is to investigate the psychometrics of the 12 items of an instrument assessing activities of daily living (ADL) using an item response theory model. Methods: A total of 648 adults with physical disabilities and having difficulties in ADLs were retrieved from the 2014 Korean National Survey on People with Disabilities. The psychometric testing included factor analysis, internal consistency, precision, and differential item functioning (DIF) across categories including sex, older age, marital status, and physical impairment area. Results: The sample had a mean age of 69.7 years old (SD = 13.7). The majority of the sample had lower extremity impairments (62.0%) and had at least 2.1 chronic conditions. The instrument demonstrated unidimensional construct and good internal consistency (Cronbach’s alpha = 0.95). The instrument precisely estimated person measures within a wide range of theta values (−2.22 logits < θ < 0.27 logits) with a reliability of 0.9. Only the changing position item demonstrated misfit (χ² = 36.6, df = 17, p = 0.0038), and the dressing item demonstrated DIF on the impairment type (upper extremity/others, McFadden’s Pseudo R² > 5.0%). Conclusions: Our findings indicate that the dressing item would need to be modified to improve its psychometrics. Overall, the ADL instrument demonstrates good psychometrics, and thus, it may be used as a standardized instrument for measuring disability in rehabilitation contexts. However, the findings are limited to adults with physical disabilities. Future studies should replicate psychometric testing for survey respondents with other disorders and for children.