Monitoring mortality

doi:10.1136/bmj.b1745

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Paul Aylin, clinical reader in epidemiology and public health1,
Alex Bottle, lecturer in medical statistics1,
Brian Jarman, emeritus professor1

¹Dr Foster Unit at Imperial, Department of Primary Care and Social Medicine, Imperial College London, Jarvis House, London EC1A 9LA

p.aylin{at}imperial.ac.uk

Mohammed and colleagues suggest that hospital standardised mortality ratios are prone to the “constant risk fallacy” and that the use of certain variables (the Charlson comorbidity index and emergency admission) for the case mix adjustment model is “unsafe.”1 2

They focus on at least two mechanisms that might contribute to this constant risk fallacy: differential measurement error, and inconsistent proxy measures of risk. Certainly, measurement error, including poor coding, will have an impact on the ratios. However, it is the extent to which they are affected which is important. The paper gives a hypothetical example of how differential measurement error can distort a standardised mortality ratio. This is an extreme example based on artificial data.

We calculated 2007-8 hospital standardised mortality ratios with and without adjustment for comorbidity (using …