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Terminology 101: Blinding in RCTs
Mar 02, 2015, By: Maher M. El-Masri, RN, PhD

Blinding: The act of concealing or masking the true intervention of a randomized clinical trial from study participants and/or clinicians and data assessors to eliminate measurement bias

Source: Guyatt, G., Rennie, D., Meade, M. O., & Cook, D. J. (Eds.). (2008). Users’ Guides to the Medical Literature: A Manual for Evidence-Based Clinical Practice (2nd ed.). New York: McGraw-Hill

Blinding refers to the act of masking the nature of the treatment that participants in a randomized controlled trial (RCT) receive. Participants receive coded treatments that all look, feel and smell the same. For participants in the intervention group the treatment contains the active ingredient (such as the drug or behavioural intervention under study) whereas for those in the control group it does not. While allocation concealment and randomization (discussed in earlier columns) minimize selection and confounding bias, respectively, blinding minimizes bias in the assessment of the true effect of the intervention (measurement bias). Blinding of participants reduces non-adherence and attrition among participants in the control group, while blinding of researchers and intervention administrators reduces the risk of exaggerating the treatment effect among those who receive the intervention.

Although blinding may not be feasible in all RCTs, it is especially important that it be implemented when the outcome is subjective (e.g., pain or energy level). For instance, if patients participating in a pain management RCT know that they are receiving the experimental therapy, they are likely to report a benefit even if they have not actually experienced one. Similarly, participants in the control group who know that they are receiving the sham or placebo treatment are more likely than those in the intervention group to not adhere to the therapy or to leave the study altogether. Either of these scenarios could bias our assessment of the true effect of the proposed pain management therapy. If the researchers and data assessors are not blinded, any preconceived assumptions they may have about the topic could potentially introduce measurement bias.

RCTs are classified into four types on the basis of their level of blinding: open label, single blind, double blind and triple blind. Open-label RCTs employ no blinding and are thus the most susceptible to measurement bias. Open-label RCTs should only be conducted if blinding is deemed to be impossible, such as in comparisons of medical and surgical interventions: it is of course not ethical to subject patients to sham surgeries. In single-blind RCTs the nature of the treatment is concealed from either the study participants or the research team, whereas in double-blind RCTs it is concealed from both the participants and the researchers, including those who administer the treatment. Triple-blind studies entail concealing the nature of the treatment from participants, researchers and administrators of the treatment, and data analysts. In triple-blind studies, data are analyzed by codes to prevent data analysts from introducing judgment bias because of their knowledge of group assignments.

The nature of the intervention determines the level of blinding possible. Readers of RCT reports should keep in mind that the lower the level of blinding, the more prone the results will be to measurement bias and thus the less confidence we should have in the generalizability of the results. resources on this topic


  • Crosby, R. A., DiClemente, R. J., & Salazar, L. F. (Eds.). (2006). Research Methods in Health Promotion.
  • Maltby, J., Williams, G. A., McGarry, J., & Day, L. (2010). Research Methods for Nursing and Healthcare.
  • Supino, P. G., & Borer, J. S. (Eds.). (2012). Principles of Research Methodology: A Guide for Clinical Investigators.
  • Webb, C., & Roe, B. (Eds.). (2008). Reviewing Research Evidence for Nursing Practice: Systematic Reviews

Maher M. El-Masri, RN, PhD, is a full professor and research chair in the faculty of nursing, University of Windsor, in Windsor, Ont.