Problems in assessing rates of infection with methicillin resistant Staphylococcus aureus

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Abstract
One of the core standards set by the Department of Health is to achieve year on year reductions in rates of infection with methicillin resistant Staphylococcus aureus (MRSA).1 This was clarified in November 2004 by the (then) health secretary John Reid, who said that he expected “MRSA bloodstream infection rates to be halved in our hospitals by 2008,” that “NHS Acute Trusts will be tasked with achieving a year on year reduction,”2 and that such a target was “achievable, measurable, and not too burdensome.” Several problems can arise, however, when measuring change in rates, particularly when the observed number of events is fairly low. These include the effects of chance variability, regression to the mean, and low power to detect genuine underlying changes. These problems are accentuated with an infectious disease, since cases tend to cluster and hence rates are “over-dispersed” relative to chance variation.3 So how should we interpret government targets on MRSA infections?