How important is it to avoid indices of deprivation that include health variables in analyses of health inequalities?

OBJECTIVES: This study aimed to quantify the difference in mortality inequalities using the Scottish Index of Multiple Deprivation (SIMD) and the Income and Employment Index (IEI; a subindex of SIMD, which excludes health) as ranking measures in Scotland. STUDY DESIGN: This ecological study was a cross-sectional analysis of routine administrative data. METHODS: Data from the 2020 SIMD and the subindex using data from only the Income and Employment domains, the IEI, were obtained. The correlation between data zones, percentage of data zones that changed deprivation tenth and differences in the Slope Index of Inequality (SII) and Relative Index of Inequality (RII) for Standardised Mortality Ratios (SMRs) across tenths were compared when data zones were ranked by SIMD and IEI. RESULTS: There was a close correlation between data zones ranked by SIMD and IEI (R2 = 0.96). When data zones were ranked by IEI, 18.7% of data zones moved to a lower deprivation tenth, and 20.8% of data zones moved to a higher deprivation tenth, compared with SIMD. However, only a negligible number of data zones moved two or more tenths. The SMRs across deprivation tenths were very similar between the SIMD and IEI, as were the summary health inequality measures of SII (87.3 compared with 85.7) and RII (0.88 and 0.86). CONCLUSION: Although there is a logical problem in using deprivation indices that include health outcomes to rank areas to calculate the scale of health inequalities, the impact of using an alternative subindex containing only data from the income and employment domains is minimal. For population-wide analyses of health inequalities in Scotland, the SIMD does not introduce a substantial bias in the health inequalities summary measures despite substantial movement of small areas between ranked population tenths. Although not examined here, this is likely to be relevant to other similar indices across the United Kingdom.