Against a priori judgements of bad methodology: Questioning double-blinding as a universal methodological virtue of clinical trials

The feature of being ‘double blind’, where neither patients nor physicians are aware of who receives the experimental treatment, is universally trumpeted as being a virtue of clinical trials. The rationale for this view is unobjectionable: double blinding rules out the potential confounding influences of patient and physician beliefs. Nonetheless, viewing successfully double blind trials as necessarily superior leads to the paradox that very effective experimental treatments will not be supportable by best (double-blind) evidence. It seems strange that an account of evidence should make a priori judgments that certain claims can never be supported by ‘best evidence’. So far as treatments with large effects go, the claim that they are effective is highly testable and intuitively they should receive greater support from the evidence. In this paper I argue that the two potential confounders ruled out by double blinding are often not actual confounders outside placebo controlled trials of treatments with mild effects and that have subjective outcome measures.