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. 2010 Nov;65(6):720-32.
doi: 10.1093/geronb/gbp129. Epub 2010 Jan 25.

The classic measure of disability in activities of daily living is biased by age but an expanded IADL/ADL measure is not

Mitchell P LaPlante  1
Affiliations

The classic measure of disability in activities of daily living is biased by age but an expanded IADL/ADL measure is not

Mitchell P LaPlante. J Gerontol B Psychol Sci Soc Sci. 2010 Nov.

Abstract

Objectives: 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.

Discussion: 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.

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Figures

Figure 1.
Figure 1.
Guttman coefficients of scalability for two measures based on needing help with activities of daily living and IADL/ADL by age, including and excluding extreme values (responses of 0 to every item or 1 to every item). Source: National Health Interview Survey on Disability.
Figure 2.
Figure 2.
Estimated item characteristic curves and actual values for getting outside (high discrimination) and managing money (low discrimination). Source: National Health Interview Survey on Disability.
Figure 3.
Figure 3.
Test characteristic curves for needing help in activities of daily living for selected age groups. Source: National Health Interview Survey on Disability.
Figure 4.
Figure 4.
Test characteristic curves for needing help in IADL/ADL for selected age groups. Source: National Health Interview Survey on Disability.
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