Double-Blind

The feature of being "double-blind", where neither patients nor physicians are aware of who receives the experimental treatment, is almost universally trumpeted as being a virtue of clinical trials. Hence, trials that fail to remain successfully double-blind are regarded as providing inferior evidential support. The rationale for this view is unobjectionable: double-blinding rules out the potentially confounding influence of patient and physician beliefs. Nonetheless, viewing double-blind trials as necessarily superior is problematic. For one, it leads to the paradox that very effective experimental treatments will not be supported by the best evidence. If a new drug were to make even the most severe symptoms of the common cold disappear within seconds, most participants and investigators would correctly identify it as the latest wonder drug and not the control (i.e. placebo) treatment. Any trial testing the effectiveness of this wonder drug will, therefore, fail to remain double-blind. Similar problems arise for treatments such as exercise and most surgical techniques, whose nature makes them resistant to being tested in double-blind conditions. It seems strange that an account of evidence should make prior judgments that certain claims can never be supported by the 'best evidence'. The result would be different if the claims at issue were pseudoscientifically untestable. But 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 than do claims about treatments with moderate effects.