Standardised mortality ratios--the effect of smoothing ward-level results

Geographical indicators of mortality provide one of the most important means of assessing the health of populations and are particularly effective in identifying inequalities in health. Geographical mortality indicators have regularly been produced by the Office for National Statistics, but not normally for areas smaller than local authorities. In order to allow variations in mortality within local authorities to be examined, in 2006 ONS published Standardised Mortality Ratios (SMRs) for wards in England and Wales, based on deaths in 1999-2003. For mortality indicators for small populations, based on small numbers of deaths, there is however a risk that results will be unstable, making geographical patterns hard to interpret. To examine whether this problem could be overcome, methods for smoothing SMRs in time and space were considered, with conclusions published in a methodology report in 2007. This article presents results from that work, illustrating the geographical patterns in mortality that emerge following smoothing of the ward-level SMRs.