Expert consensus on contamination in educational trials elicited by a Delphi exercise

Context  Contamination occurs when participants in the control arm of a trial receive an intervention that was intended for the intervention arm. Contamination tends to bias estimates of effectiveness towards no effect, and to reduce a trial's power to detect significant differences in outcomes. This is often assumed to be a problem in trials of educational interventions because of the transferability of verbal and experiential approaches. Cluster randomisation and other trial designs and analyses commonly used to reduce contamination can themselves cause bias or reduce statistical power.