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Variation in hospital mortality associated with surgery.

To the Editor: The analysis of variation in hospital mortality by Ghaferi et al. (Oct. 1 issue)1 requires further scrutiny. The primary exposure variable used by these authors was risk-adjusted mortality. However, there are difficulties in applying risk-adjustment methods to compare patient outcomes across multiple hospitals; these difficulties result from risk relations and interactions that are not constant. Different methods of adjustment for severity frequently show different results with regard to the hospital rankings generated.2 In comparisons of standardized hospital mortality ratios, there is even evidence that casemix adjustment may paradoxically increase the bias.3 Moreover, the size of the hospitals, which was not stated, could be expected to affect their ranking because, as compared with the larger hospitals, smaller hospitals would disproportionately appear among the very-high-mortality and very-low-mortality quintiles. The effect of the study size on the event rate is readily apparent in a funnel plot.4 James C. Hurley, M.D., Ph.D.