Correspondence:

clinical trials double-blind method placebo effect In their paper discussing the importance of double-blinding in controlled trials, Furberg and Soliman stated that “one of the established and fundamental principles for avoiding the problem of bias is to keep the study participants and the investigators blinded, or masked, to the identity of the assigned interventions ” [1]. As a support to this argument they described the subgroup findings of Karlowski et al.’s trial, which examined the effect of vitamin C supplementation on the common cold [2,3]. Furberg and Soliman put a great weight on the importance of double-blinding, yet they are lax on other fundamental principles of controlled trials. A popular textbook of clinical trials emphasizes that “excluding randomized participants from analysis and subgrouping on the basis of outcome or response variables can lead to biased results of unknown magnitude ” [4, p. 284]. The subgroup analysis by Karlowski et al. [2,3] violates both of these principles. First, Karlowski et al. [2] excluded about half of the randomized participants from th