Measurement equivalence in ADL and IADL difficulty across international surveys of aging: findings from the HRS, SHARE, and ELSA

Abstract

Objective: To examine the measurement equivalence of items on disability across three international surveys of aging.

Method: Data for persons aged 65 and older were drawn from the Health and Retirement Survey (HRS, n = 10,905), English Longitudinal Study of Aging (ELSA, n = 5,437), and Survey of Health, Ageing and Retirement in Europe (SHARE, n = 13,408). Differential item functioning (DIF) was assessed using item response theory (IRT) methods for activities of daily living (ADL) and instrumental activities of daily living (IADL) items.

Results: HRS and SHARE exhibited measurement equivalence, but 6 of 11 items in ELSA demonstrated meaningful DIF. At the scale level, this item-level DIF affected scores reflecting greater disability. IRT methods also spread out score distributions and shifted scores higher (toward greater disability). Results for mean disability differences by demographic characteristics, using original and DIF-adjusted scores, were the same overall but differed for some subgroup comparisons involving ELSA.

Discussion: Testing and adjusting for DIF is one means of minimizing measurement error in cross-national survey comparisons. IRT methods were used to evaluate potential measurement bias in disability comparisons across three international surveys of aging. The analysis also suggested DIF was mitigated for scales including both ADL and IADL and that summary indexes (counts of limitations) likely underestimate mean disability in these international populations.

Figure 1.

Item characteristic curves for Health and Retirement Survey (HRS) and Survey of Health, Ageing and Retirement in Europe (SHARE; statistically significant items).

Figure 2.

Item characteristic curves for Health and Retirement Survey (HRS) and English Longitudinal Study of Aging (ELSA; statistically significant items).

Figure 3.

Overall impact of differential item functioning (DIF): Survey of Health, Ageing and Retirement in Europe (SHARE) and English Longitudinal Study of Aging (ELSA) compared with Health and Retirement Survey (HRS).

Figure 4.

Activities of daily living (ADL) and instrumental activities of daily living (IADL) summary scores 1–11, differential item functioning (DIF)–adjusted item response theory (IRT), IRT not DIF adjusted, and original.

Figure 5.

Distribution of rescaled item response theory (IRT) scores, both adjusted and unadjusted for four original summed activities of daily living (ADL)/instrumental activities of daily living (IADL) scores.