Blinding (or masking) in RCTs is the process of withholding information about treatment allocation from those who could potentially be influenced by this information. Blinding has long been considered an important safeguard against bias. Benjamin Franklin, in 1784, was probably the first to use blinding in scientific experimentation.2 Louis XVI commissioned Franklin to evaluate mesmerism, the most popular unconventional "healing fluid" of the eighteenth century.2 By applying a blindfold to participants Franklin removed their knowledge of when mesmerism was and was not being applied. Blinding eliminated the intervention's effects and established mesmerism as a sham.2 From this work, the scientific community recognised the power of blinding to enhance objectivity, and it quickly became, and remains, a commonly used strategy in scientific research.
Individuals in the 7 groups in the table are likely to have, or to develop, opinions about the efficacy of the intervention being investigated. Because of these opinions, unblinded individuals can systematically bias trial findings through conscious or unconscious mechanisms. When unblinded, participants may introduce bias through use of other effective interventions, differential reporting of symptoms,3 psychological or biological effects of receiving a placebo (although recent studies show conflicting evidence),4, 5 or dropping out. Unblinded healthcare providers can distort trial results if they differentially prescribe effective co-interventions, influence compliance with follow up, or influence patient reports.3 Unblinded data collectors can introduce bias through differential encouragement during performance testing, differential timing or frequency of outcome measurements, and variable recording of outcomes.6, 7 Unblinded judicial assessors may introduce bias in their assessments of outcomes, this being most likely during assessment of subjective outcomes.3 Unblinded data analysts have the potential to introduce systematic bias through decisions on patient withdrawals, post hoc selection of outcomes or analytic approaches, selection of time points that show the maximum or minimum effects, and many other decisions.8 Unblinded members of the data safety and monitoring committee may introduce bias at the time of interim analyses through their recommendations to stop or continue a study.8 Blinding of authors, while seldom done,8, 9 may reduce the bias in the presentation and interpretation of results.
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Evidence-Based Mental
Health 2002; 5:36-37
© 2002 Evidence-Based
Mental Health
EBMH notebook |
Double blind, you have been voted off the island!
P J Devereaux, MD1, Mohit Bhandari, MD, MSc1, Victor M Montori, MD, MSc2, Braden J Manns, MD3, William A Ghali, MD, MPH3 and Gordon H Guyatt, MD, MSc4
1 McMaster University
Hamilton, Ontario, Canada
2 Mayo Clinic
Rochester, Minnesota, USA
3 University of Calgary
Calgary, Alberta, Canada
4 McMaster University
Hamilton, Ontario, Canada