Self-reporting quality of life in mild-to-moderate Alzheimer's disease and Lewy body dementia: Comparing capability and health-focused measures using response process validation

Background:

Capability measures offer theoretical advantages for assessing wellbeing in health economics and outcomes research, yet their feasibility for self-reporting in dementia remains unclear. This study compares the feasibility of self-reporting on capability versus health-focused quality of life measures among people living with mild-moderate dementia, using response process validation.

Methods:

Twenty-three community-dwelling participants with Alzheimer's disease (n=11) or Lewy body dementia (n=12) completed four measures using concurrent think-aloud cognitive interviewing: two capability measures (ICECAP-O, ICECAP-SCM) and two health-focused measures (QoL-AD, and AQoL-4D). Errors were coded using Tourangeau's four-stage response model (comprehension, retrieval, judgement, response). Completion times, support requirements, and error patterns were compared across measures and diagnostic groups.

Results:

Capability measures showed higher error rates (13.7-18.3%) than health-related measures (5.1-8.4%) despite shorter completion times. Comprehension errors (51.2% of all errors) were most common across all measures, particularly for abstract concepts like 'dignity' and 'doing things that make you feel valued'. Retrieval errors were rare (4.9%), suggesting memory is not the primary barrier. Diagnostic groups showed distinct error profiles: Lewy body dementia participants had more comprehension errors (57.4% vs 42.9%), whilst Alzheimer's disease participants showed more response-selection difficulties (25.7% vs 14.9%).

Conclusions:

While self-reporting remains feasible in mild-to-moderate Alzheimer's disease and Lewy body dementia, capability measures pose greater cognitive demands than health-focused measures, primarily due to comprehension of abstract concepts and distinguishing capability from functioning rather than memory retrieval. Measures should be validated for specific dementia subtypes, and hybrid approaches anchoring capability judgements in concrete experiences warrant exploration. This exploratory work contributes response process validity evidence to inform measure selection in dementia research and clinical practice, ensuring people living with dementia can meaningfully participate in health economics and policy decisions that affect their care.