Double-blind
A double-blind trial keeps both participants and investigators, usually including those who assess outcomes, unaware of who received the intervention and who received the comparison, so that this knowledge cannot sway the results.
What it means in practice
Blinding means keeping participants, investigators (usually the care providers) or assessors (the people collecting outcome data) unaware of the assigned intervention, so that the knowledge cannot influence them [1]. Double blinding usually refers to all three groups [1].
In practice the label is loose, because terms such as single, double and triple blind mean different things to different people [1]. In a survey of 91 internal medicine physicians at 3 Canadian university centers, respondents gave 17 different interpretations of double blinding, and 25 textbooks offered 9 different definitions [2]. The most common physician interpretation was held by only 38% of respondents [2]. When the authors of 200 trials reported as double blind were surveyed, they described 18 different combinations of who was actually blinded, and about one in five had not blinded participants, care providers or data collectors [3].
Why it matters when reading nootropics research
Blinding usually reduces biased assessment of outcomes and can also help keep participants in the trial [1]. In an analysis of 146 meta-analyses, trials with subjective outcomes that lacked blinding showed exaggerated effect estimates (ratio of odds ratios 0.75), while trials with objective outcomes showed little evidence of bias [4]. Later research on the average effect of blinding has been more mixed; see Open-label.
Schulz and Grimes advise that a trial that is not double-blind should not automatically be judged inferior, and that reports should state exactly who was blinded and how [1]. When a paper claims blinding without that detail, they recommend that readers stay skeptical about how much bias it removed [1]. On a supplement study, the methods section is the place to check who was kept unaware.
References
- Schulz KF, Grimes DA. Blinding in randomised trials: hiding who got what. Lancet. 2002.
- Devereaux PJ, Manns BJ, Ghali WA, et al. Physician interpretations and textbook definitions of blinding terminology in randomized controlled trials. JAMA. 2001.
- Moher D, Hopewell S, Schulz KF, et al. CONSORT 2010 explanation and elaboration: updated guidelines for reporting parallel group randomised trials. BMJ. 2010.
- Wood L, Egger M, Gluud LL, et al. Empirical evidence of bias in treatment effect estimates in controlled trials with different interventions and outcomes: meta-epidemiological study. BMJ. 2008.