Testing and evaluating updated variants of the rurality index for Japan: a nationwide methodological study

OBJECTIVES: To update the rurality index for Japan (RIJ) using the most recent national data and to test and evaluate the updated RIJ variants that incorporate alternative distance metrics and a modified classification of remote islands, thereby providing methodological guidance for their use in research and health policy. DESIGN: Nationwide methodological study. SETTING: Japan. PARTICIPANTS: All postal code areas in Japan; analyses were aggregated into municipalities and secondary medical care areas for evaluation. EXPOSURE: Six RIJ variants ('RIJ family') constructed by combining three distance metrics (direct distance, road-based distance and travel time by car) and two island classifications (original RIJ and modified RIJ), standardised to a continuous 0-100 scale using percentile ranks. PRIMARY OUTCOME MEASURES: Concordance among RIJ variants was assessed using Spearman's rank correlation coefficients, quintile reclassification matrices and rank-difference analyses; convergent validity was evaluated using the national physician distribution index. Criterion-related validity was assessed using municipal-level life expectancy for men and women. RESULTS: All RIJ variants had extremely high concordance (Spearman's ρ≥0.99 across all pairwise comparisons). Compared with the previous RIJ classification, >98% of the geographic units remained in the same rurality quintile and