Associations between Postcode-based Area Deprivation Index and Individual-level Socioeconomic Status: Findings from Two Population-based Cohort Studies.

BACKGROUND: The Area Deprivation Index (ADI) is widely used to monitor health inequalities and often used as a proxy for individual socioeconomic status (SES). However, the extent to which it reflects individual SES remains unclear. This study aimed to establish evidence linking neighborhood-level ADI with individual SES indicators in Japan and examine whether similar patterns of association are observed across independent population-based cohorts. METHODS: We analyzed baseline data from two population-based cohorts: the Japan Public Health Center-based Prospective Study for the Next Generation (JPHC-NEXT) (baseline survey, 2011-2016; ages 40-74 years; N=100,024) and the Yokohama Health Study (YHS) (baseline survey, 2023; ages 30-69 years; N=8,860). ADI was ranked into quartiles (Q1: lowest, Q4: highest deprivation) based on the distribution across all postcodes in Japan and linked to each participant by residential area. Logistic regression models estimated associations between ADI quartile and low income (<1.53 million JPY) or limited education (≤9 or ≤12 years) by gender. RESULTS: Higher ADI showed increasing trends of association with low income and ≤9 years of education in both cohorts and across genders, with p for trend <0.001 for all. In contrast, associations with ≤12 years of education were less consistent across cohorts and subgroups, with no significant trend observed among women in JPHC-NEXT (p for trend = 0.74). CONCLUSIONS: While the ADI consistently reflected income-related disadvantage across cohorts, its association with educational attainment varied across contexts. This should be carefully considered when using or interpreting ADI-based measures of health inequality in Japan.