- •Blinding in clinical trials has the potential to reduce bias.
- •There is a live yet underreported debate about the value of measuring blinding success.
- •Interpreting the success of blinding can be problematic and potentially misleading.
- •However, failure to report the success of blinding, if it is measured, seems like willful withholding of information that is at least potentially useful.
- •We suggest a middle road whereby the success of blinding be measured (where feasible) and interpreted with caution.
Purchase one-time access:Academic & Personal: 24 hour online accessCorporate R&D Professionals: 24 hour online access
One-time access price info
- For academic or personal research use, select 'Academic and Personal'
- For corporate R&D use, select 'Corporate R&D Professionals'
- The CONSORT statement: revised recommendations for improving the quality of reports of parallel-group randomised trials.Lancet. 2001; 357: 1191-1194
- CONSORT 2010 Explanation and Elaboration: updated guidelines for reporting parallel group randomised trials.BMJ. 2010; 340: c869
- CONSORT 2010 changes and testing blindness in RCTs.Lancet. 2010; 375: 1144-1146
- Fool's gold? Why blinded trials are not always best.BMJ. 2020; 368: l6228
- TIDieR-Placebo: A guide and checklist for reporting placebo and sham controls.PLoS Med. 2020; 17e1003294https://doi.org/10.1371/journal.pmed.1003294
Questioning Double Blinding as a Universal Methodological Virtue of Clinical Trials: Resolving the Philip's Paradox. Philos Evidence-Based Med:63-79.
- Who is blinded in randomized clinical trials? A study of 200 trials and a survey of authors.Clin Trials. 2006; 3: 360-365
- Active placebos versus antidepressants for depression.Cochrane Database Syst Rev. 2004; Cd003012
- The matching quality of experimental and control interventions in blinded pharmacological randomised clinical trials: a methodological systematic review.BMC Med Res Methodol. 2016; 16: 18
- Commentary: Measuring the success of blinding in RCTs: don't, must, can't or needn't?.Int J Epidemiol. 2007; 36: 664-665
- Blinding in randomised trials: hiding who got what.Lancet. 2002; 359: 696-700
- Hypnosis, reporting bias, and suggested negative hallucinations.J Abnorm Psychol. 1992; 101: 192-199
- Observer bias in randomized clinical trials with measurement scale outcomes: a systematic review of trials with both blinded and nonblinded assessors.Can Med Assoc J. 2013; 185 (E201-E11)
- Practical tips for surgical research: blinding: who, what, when, why, how? Canadian journal of surgery.Journal canadien de chirurgie. 2010; 53: 345-348
- Impact of blinding on estimated treatment effects in randomised clinical trials: meta-epidemiological study.BMJ. 2020; 368: l6802
- Empirical Evidence of Study Design Biases in Randomized Trials: Systematic Review of Meta-Epidemiological Studies.PLoS One. 2016; 11e0159267
- Empirical evaluation of which trial characteristics are associated with treatment effect estimates.J Clin Epidemiol. 2016; 77: 24-37
- Influence of blinding on treatment effect size estimate in randomized controlled trials of oral health interventions.BMC Med Res Methodol. 2018; 18: 42
- Blinding in Physical Therapy Trials and Its Association with Treatment Effects: A Meta-epidemiological Study.Am J Phys Med Rehabil. 2017; 96: 34-44
- Observer bias in randomised clinical trials with binary outcomes: systematic review of trials with both blinded and non-blinded outcome assessors.BMJ. 2012; 344: e1119
- Observer bias in randomized clinical trials with measurement scale outcomes: a systematic review of trials with both blinded and nonblinded assessors.CMAJ. 2013; 185: E201-E211
- Observer bias in randomized clinical trials with time-to-event outcomes: systematic review of trials with both blinded and non-blinded outcome assessors.Int J Epidemiol. 2014; 43: 937-948
- Bias due to lack of patient blinding in clinical trials. A systematic review of trials randomizing patients to blind and nonblind sub-studies.Int J Epidemiol. 2014; 43: 1272-1283
- Influence of reported study design characteristics on intervention effect estimates from randomized, controlled trials.Ann Intern Med. 2012; 157: 429-438
- Re: Impact of blinding on estimated treatment effects in randomised clinical trials: meta-epidemiological study.BMJ. 2020; 368: l6802
- In search of justification for the unpredictability paradox.Trials. 2014; 15: 480
- The unpredictability paradox: review of empirical comparisons of randomised and non-randomised clinical trials.BMJ. 1998; 317: 1185-1190
- Statins for the primary prevention of cardiovascular disease.Cochrane Database Syst Rev. 2013; 1CD004816
- Ascorbic acid for the common cold: a prophylactic and therapeutic trial.JAMA. 1975; 231: 1038-1042
- Duration of symptoms and plasma cytokine levels in patients with the common cold treated with zinc acetate: a randomized, double-blind, placebo-controlled trial.Ann Intern Med. 2000; 133: 245-252
- Failure of zinc gluconate in treatment of acute upper respiratory tract infections.Antimicrob Agents Chemother. 1989; 33: 646-648
- A review of blinding in randomized controlled trials found results inconsistent and questionable.J Clin Epidemiol. 2005; 58: 1220-1226
- Assessment of blinding in clinical trials.Controlled Clin Trials. 2004; 25: 143-156
- An index for assessing blindness in a multi-centre clinical trial: disulfiram for alcohol cessation–a VA cooperative study.Stat Med. 1996; 15: 1421-1434
- Testing for blinding in sham-controlled studies for procedural interventions: the third-party video method.CMAJ. 2019; 191: E272-E2e3
- Blinded trials taken to the test: an analysis of randomized clinical trials that report tests for the success of blinding.Int J Epidemiol. 2007; 36: 654-663
- Turning a blind eye: the success of blinding reported in a random sample of randomised, placebo controlled trials.BMJ. 2004; 328: 432
- A randomized trial of aspirin and sulfinpyrazone in threatened stroke.N Engl J Med. 1978; 299: 53-59
- Turning a blind eye: Authors have blinkered view of blinding.BMJ. 2004; 328: 1135-1136
- Towards a proposal for assessment of blinding success in clinical trials: up-to-date review.Commun Dent Oral Epidemiol. 2009; 37: 477-484
- Does different information disclosure on placebo control affect blinding and trial outcomes? A case study of participant information leaflets of randomized placebo-controlled trials of acupuncture.BMC Med Res Methodol. 2018; 18: 13
- Does aerobic training alleviate fatigue and improve societal participation in patients with multiple sclerosis? A randomized controlled trial.Multiple Sclerosis (Houndmills, Basingstoke, England). 2017; 23: 1517-1526
- Changes in beliefs identify unblinding in randomized controlled trials: a method to meet CONSORT guidelines.Contemp Clin Trials. 2005; 26: 25-37
Declarations of Competing interests: None.