The classic measure of disability in activities of daily living is biased by age but an expanded IADL/ADL measure is not.
OBJECTIVE
To evaluate, by age, the performance of 2 disability measures based on needing help: one using 5 classic activities of daily living (ADL) and another using an expanded set of 14 activities including instrumental activities of daily living (IADL), walking, getting outside, and ADL (IADL/ADL).


METHODS
Guttman and item response theory (IRT) scaling methods are used with a large (N = 25,470) nationally representative household survey of individuals aged 18 years and older.


RESULTS
Guttman scalability of the ADL items increases steadily with age, reaching a high level at ages 75 years and older. That is reflected in an IRT model by age-related differential item functioning (DIF) resulting in age-biased measurement of ADL. Guttman scalability of the IADL/ADL items also increases with age but is lower than the ADL. Although age-related DIF also occurs with IADL/ADL items, DIF is lower in magnitude and balances out without causing age bias.


CONCLUSIONS
An IADL/ADL scale measuring need for help is hierarchical, unidimensional, and unbiased by age. It has greater content validity for measuring need for help in the community and shows greater sensitivity by age than the classic ADL measure. As demand for community services is increasing among adults of all ages, an expanded IADL/ADL measure is more useful than ADL.
LaPlante
Mitchell P
MP
The Institute for Health & Aging, 3333 California Street, Room 340, University of California San Francisco, San Francisco, CA 94118, USA. mitch.laplante@ucsf.edu
D000203:Activities of Daily Living; D000293:Adolescent; D000328:Adult; D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D015982:Bias; D003627:Data Interpretation, Statistical; D004185:Disability Evaluation; D006306:Health Surveys; D006801:Humans; D008875:Middle Aged; D014481:United States; D055815:Young Adult