The Rurality Index for Japan (RIJ) was employed as an indicator of rurality [18]. The RIJ consists of four factors: population density, distance to secondary or tertiary care hospitals, remote islands, and heavy snow areas [18]. The score is calculated by adding each factor with weight and describes rurality from 1 to 100 for all zip codes in Japan (with 100 being the most rural) [18]. The RIJ showed good validity in the validation study [18].
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