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The considerable between patient variability in the trial of Example 1.7 is illustrated in Figure 1.1. In the 41 patients receiving Azathioprine, the reduction in disease activity (SASSAD) ranged from −10 to 32. There is considerable overlap of these values with those from the 20 patients receiving Placebo whose values range from −12 to 20. This figure clearly illustrates that, although there is considerable variation, the majority of patients in both groups improve. Further, the corresponding reduction in percentage body area affected with Azathioprine was reported to range from approximately −15 to 85% and for placebo approximately −20 to 45%. Nevertheless, even with the majority of patients improving in both groups, the trial of Meggitt, Gray and Reynolds (2006) indicated a better outcome, on average, for those receiving Azathioprine.

Figure 1.1 Individual patient reductions in disease activity (SASSAD) for the Azathioprine and Placebo treatment groups with the corresponding means indicated.

Source: Data from Meggitt, Gray and Reynolds (2006).

With such variability, it follows that, in any comparison made in a biomedical context, differences between subjects or groups of subjects frequently occur. These differences may be due to real effects, random variation or both. It is the job of the experimenter to decide how this variation should be taken note of in the design of the ensuing trial. The purpose being that, once at the analysis stage, the variation can be partitioned suitably into that due to any real effect of the interventions on the difference between groups and that from the random or chance component.