Rasch analysis of the International Wellbeing Index in older adults.

BACKGROUND: The International Wellbeing Index is a measure of general quality of life formed by two scales: the Personal Wellbeing Index (PWI) and the National Wellbeing Index (NWI). This paper studies the psychometric properties of the PWI and NWI, using Rasch analysis and classic psychometric methods.

METHODS: The PWI and NWI were applied to a representative sample of 1106 community-dwelling adults, aged 60 years and over, residing in Spain. Mean ± standard deviation age was 72.07 ± 7.83 years and 56.3% were women.

RESULTS: Five PWI items (achieving in life, relationships, safety, community connectedness, and future security), and five NWI items (economic situation, state of environment, social conditions, business, and national security of the country) fitted the Rasch model. After adjusting the response scale format, satisfactory fit was obtained, with good reliability (person separation index of 0.91 for both the PSI and NWI), local independency of items, and strict unidimensionality. The measures showed adequate external construct validity with related measures.

CONCLUSIONS: The PWI and the NWI, with fewer items and simpler response scale formats, provided valid and reliable linear measures in older adults, according to Rasch and classic psychometric analyses.