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Many major trials study time to a primary event outcome. For instance, the Evaluation of XIENCE versus Coronary Artery Bypass Surgery for Effectiveness of Left Main Revascularization (EXCEL) trial studied a composite ischemic endpoint: death from any cause, myocardial infarction, or stroke over three year follow‐up [3].

A Kaplan–Meier life‐table plot is the main method of displaying such data by treatment group (Figure 6.2). It displays the cumulative percentage of patients experiencing the event over time for each group. This method takes account of patients having different lengths of follow‐up (e.g. any lost to follow‐up before the intended three years).

Figure 6.2 Kaplan–Meier life‐table plot showing pattern of treatment difference over time (e.g. EXCEL 3‐year follow‐up).

Such a plot is a useful descriptive tool, but one needs to use a logrank test to see if there is evidence of a treatment difference in the incidence of events. For instance, the PCI (n = 948) and coronary artery bypass grafting (CABG) alone (n = 957) groups had composite ischemia in 14.5% and 14.1% of patients, respectively. The log‐rank test uses the total data by group displayed to obtain p = 0.98 (i.e. the data are consistent with the null hypothesis of no treatment difference). The log‐rank test can be thought of as an extension, indeed improvement, to the simpler chi‐squared test comparing two percentages because it takes into account the fact that patients have been followed for, or deaths occur at, differing times from randomization.

With time to event data, the hazard ratio is used to estimate any relative treatment differences in risk. It is similar to, but more complicated to calculate, than the simple relative risk already mentioned. It effectively averages the instantaneous relative risk occurring at different follow‐up times, using what is commonly called a Cox proportional hazards model. In this case the hazard ratio comparing PCI with CABG is 1.00 with 95% CI 0.79–1.26. Thus, there is no increase in hazard, but even if the hazard ratio was different than 1, if the 95% CI includes 1 there is no statistical significance in the hazard between the two groups. For instance, the hazard ratio for death from any cause at three year follow‐up in the same trial is 1.34 with 95% CI 0.94–1.91 [3]. Even if there is an observed 34% increase in hazard, the 95% CI includes 1, reflecting lack of statistical significance.