Detection of changes in mortality after heart surgery

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# Control limits failed to account for case mix {#article-title-2} EDITOR—We are concerned about the graphical technique described by Poloniecki et al in their analysis of perioperative mortality rates associated with cardiac surgery.1 Figure 2 shows three traces: observed mortality performance bracketed by control limits and plotted against the number of successive cases performed. The interpretation of the middle of the traces is straightforward since it is simply a variable life adjusted display that has previously been described and will be familiar to many cardiac surgeons in the United Kingdom.2 The use of control limits, on the other hand, is new. However, the usefulness and indeed the validity of these is not clear. As the authors themselves note, their analysis does not amount to a formal test of significance since the control limits have not been corrected for multiple testing; this is a major deficiency. The use of 99% control limits rather than 95% control limits presumably increases their separation and makes them …