Contamination may mask the effectiveness of an intervention in an otherwise perfectly conducted trial. But what is contamination? The CLAHRC WM Director proposes that we do away with this moniker and instead use the term ‘masking’ to describe circumstances where improvement in the control groups vitiates a positive result. He further proposes that ‘masking’ be categorised into three classes:
- Mimetic masking, where the control group becomes aware of the study intervention and implements it.
- Neighbourhood masking, where the control group benefits from any benefit in the intervention group through the ‘neighbourhood effect’. Herd immunity is an obvious example. Another recent example relates to the effects of deworming, which reduces the load of eggs that may infect others. In both cases the controls, rather than being ‘contaminated’ are ‘de-contaminated’!
- Temporal masking, where the intervention, or another effective intervention, comes into vogue quite independently of the study and hence where headroom for further gains in the intervention group are diminished. This has also been referred to as a ‘rising tide phenomenon’.
Trials may be conceptualised as having pragmatic or aetiological motivations. Categories 1 and 2 undermine both the pragmatic and aetiological motivations for a trial. Category 3 undermines only aetiological. Category 2, and to a variable extent 1, may be avoided by cluster studies, but this does not mitigate Category 3 masking.
— Richard Lilford, CLAHRC WM Director
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