Explanatory and pragmatic estimates of the treatment effect when deviations from allocated treatment occur

In a randomized clinical trial comparing two treatments it can happen that certain patients receive the treatment other than that determined by random allocation. Significance testing is usually performed by 'intention to treat', that is, comparison of groups as determined by random allocation. The difference between the mean values of the outcome variable for these two groups estimates the difference in practical use between two treatment policies, corresponding to a pragmatic approach. An attenuation factor can then be used to obtain an estimate of the underlying difference in effectiveness between the two treatments, corresponding to an explanatory paradigm. Thus two distinct estimates are obtained; in many instances both are valid, but have distinct interpretations. Correspondingly, in planning sample size requirements when deviations from allocated treatment can be anticipated, the target treatment difference may be understood in either an explanatory or a pragmatic sense; for the sample size assessment method it is necessary to take this distinction into account.