Pragmatic and internal validity issues in sampling in caregiver studies: a comparison of population-based, registry-based, and ancillary studies.

OBJECTIVES: Studies of caregivers illustrate a classic sampling dilemma: maximizing recruitment without compromising study validity. Because caregivers are defined in relation to a care recipient, sampling methods are often determined by pragmatic decisions such as access, efficiency, and costs. However, overlooking validity may result in selection bias, misclassification of caregiver status, and the confounding of results. Validity and pragmatic concerns were compared in four caregiver studies that used different sampling frames: community based, Alzheimer's disease registry, and ancillary studies to existing epidemiologic studies.
METHODS: Systematic comparison of validity and of pragmatic aspects of sampling frames, recruitment methods, and participation rates, with attention to caregiver identification, inclusion criteria, and sample restriction.
RESULTS: All studies used task-based inclusion criteria. Caregiver participation rates ranged from 81% to 96%, with higher rates in community-based and registry-based studies than in ancillary studies. The latter studies benefited from unbiased selection of noncaregivers. DISCUSSION: Regardless of sampling frame, standard task-based inclusion criteria to define caregivers may enhance validity.