A STANDARDIZATION APPROACH TO THE CONTROL OF SOCIOECONOMIC CONFOUNDING IN SMALL-AREA STUDIES OF ENVIRONMENT AND HEALTH

Objective - To assess how effectively a routine adjustment can be made for socioeconomic confounding in small area studies of environment an d health using indirect standardisation and small area deprivation ind ices, including analysis of the appropriate size of population unit on which to base the deprivation index and the importance of region and urban/rural status as axes of stratification. Method - Standardised mo rbidity ratios were calculated for cancers in Great Britain for 1981 a nd standardised mortality ratios for all cause mortality in Great Brit ain between 1982 and 1985. Deprivation indices were calculated for enu meration districts and wards from 1981 small area census statistics. C ancers and deaths were allocated to enumeration districts via their po stcode. Standardised morbidity and mortality ratios were calculated by quintile of enumeration district according to the deprivation index. Standardised mortality ratios were further analysed by deprivation of ward, region, and urban/rural status. Results - Strong relationships w ere found between all cause mortality and the incidence of selected ca ncers and deprivation quintile - there was up to a twofold difference in lung cancer incidence between the highest and lowest quintile. The deprivation index can be used to measure gradients of deprivation acco rding to the distance from industrial sites. The deprivation index for enumeration districts showed similar discrimination of mortality as t he index for wards. There is some interaction between deprivation and region in their effect on the standardised mortality ratios, leading t o a small bias in the estimation of expected numbers if this is not ta ken into account. The relationship between deprivation, urban/rural st atus, and mortality is complex and confounded by region, but mortality tends to be higher in urban than in rural areas within quintiles of d eprivation. Discussion - Whether calculated for enumeration districts or wards, the main problems in the interpretation of the deprivation i ndex may be its limited correlation with the risk factors of interest and its concentration on present rather than past socioeconomic status . Indirect standardisation based on stratification for deprivation and other variables involves a trade off between bias and precision in de termining the fineness and the number of axes of stratification. Some bias may occur due to interaction between region and deprivation and t he effects of urban/rural status. Complementary approaches including m odelling and proportional mortality or morbidity analyses may be neede d and the possibility of residual socioeconomic confounding must alway s be considered. Conclusion - There is potential for important socioec onomic confounding in small area studies of environmental pollution an d health where the health outcome under examination has a strong relat ionship to socioeconomic status and where the putative excess risk due to pollution may be small. One method of controlling for confounding is to use an ecological measurement of deprivation in small areas, and to adjust for deprivation by indirect standardisation. However, resid ual socioeconomic confounding can be expected, which may seriously com plicate the interpretation of small area studies.