Ethnic density and deliberate self harm; a small area study in south east London

Study objective-Relative risks are frequently used to convey how strongly o utcomes like mental illness and suicidal behaviour are associated with pers onal characteristics Like ethnic background. This study examined whether RR s for deliberate self harm (DSH) in ethnic groups vary between small areas according to their ethnic mix. Design-Small area study of DSH rates in ethnic groups, by local ethnic dens ity, using negative binomial regression. Setting-73 south London electoral wards, 1994-1997. Subjects-1643 people attending casualty after DSH. Main results-African-Caribbean and Asian DSH rates, relative to the white p opulation, varied between wards. A linear model indicated a decline by fact ors (relative rate ratios) 0.76 (95% confidence intervals (CI) 0.64 to 0.90 ) and 0.59 (95% CI 0.36 to 0.97) respectively per SD increase in the local size of these minority populations. However, for both groups, an inverted U shaped curve provided a better fit for the link between the relative DSH r ate in these groups and their local population density. Conclusions-The DSH rate of minority groups relative to the white group is low (suggesting protection) in some areas, and high (suggesting risk) elsew here. This has implications for management of suicidal behaviour in ethnic groups but also for interpretation, and policy implementation, of research on risk for suicidal behaviour, and, probably, many other outcomes. Relativ e risks or rates are not stable indicators of association in psychiatric ep idemiology.