Look out for intervention comparisons where subject’s outcomes were not counted in the group to which they were assigned. Subjects’ outcomes should be analysed in their original groups.
Randomized allocation helps to ensure that the comparison groups have similar characteristics. However, sometimes the subject’s do not receive the allocated interventions. The characteristics of such subject’s often differ from those that do receive the interventions as allocated. Excluding from the analysis subjects that did not receive the allocated intervention may mean that like is no longer being compared with like. “Contamination” may lead to an underestimate of effect relative to what would have happened if every subject had received what was intended.
REMEMBER: Be cautious about relying on the results of intervention comparisons if subject outcomes are not counted in the group to which they were allocated.