Masking Study Participants

  • Lea DryeEmail author
Living reference work entry


Masking or blinding in clinical trials refers to the process of keeping the identity of the assigned treatment hidden from specific groups of individuals such as participants, study staff, or outcome assessors. The purpose of masking is to minimize conscious and unconscious bias in the conduct and interpretation of a trial. Masking participants in clinical trials is a key methodological procedure since patient expectations can introduce bias directly through how a participant reports patient-reported outcomes but also indirectly through his or her willingness to participate in and adhere to study activities.

The complexity of operational aspects of masking participants is often underestimated. Masking is facilitated by placebos, dummies, sham devices, or sham procedures/surgeries. The success of masking depends on how closely the placebo or sham matches the active treatment. Creation of a completely identical placebo is generally possible only when active drug and matching placebo are provided by the manufacturer. Masking of participants becomes more complicated if there are more than two experimental treatment groups, an active control, if treatments are taken at different intervals or via different routes, or if sham devices or procedures are required.

Trials in which participants are masked should have procedures in place to unmask. Most unmasking is routine unmasking in which investigators communicate treatment assignment with participants after treatment and follow-up are complete. In addition to this routine unmasking, masked trials should have procedures to immediately unmask at any hour of the day in the event of an emergency.


Blind Mask Single mask Double mask Unmask Placebo Sham 


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Copyright information

© Springer Nature Switzerland AG 2019

Authors and Affiliations

  1. 1.Office of Clinical AffairsBlue Cross Blue Shield AssociationChicagoUSA

Section editors and affiliations

  • O. Dale Williams
    • 1
  1. 1.Department of MedicineUniversity of AlabamaBirminghamUSA

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