Blinding (masking)

Blinding (masking) is a clinical trial design procedure in which knowledge of treatment assignment is withheld from one or more parties involved in the trial. It is used to limit bias in trial conduct and interpretation.

Definitions

ICH E6(R3) Guideline

A procedure in which one or more parties to the trial are kept unaware of the treatment assignment(s). Single-blinding usually refers to the participant(s) being unaware, and double-blinding usually refers to the participant(s) and investigator(s) and, if appropriate, other investigator site staff or sponsor staff being unaware of the treatment assignment(s) 1.

Purpose and scope

Blinding is intended to limit bias arising from knowledge of treatment assignment. Such knowledge may influence participant recruitment and allocation, subsequent care, participant attitudes toward treatment, endpoint assessment, handling of withdrawals, and exclusion of data from analysis.

Single-blind trial

According to ICH E9, a single-blind trial is one in which the investigator and/or investigator site staff know the treatment assignment, but the participant does not, or vice versa 2. The term therefore covers different arrangements, depending on which parties are blinded.

Double-blind trial

According to ICH E9, a double-blind trial is one in which neither the participant nor investigator or sponsor staff involved in treatment or clinical evaluation know the treatment assignment. This includes personnel responsible for determining participant eligibility, evaluating endpoints, or assessing protocol compliance.

ICH E9 identifies double-blinding as the preferred approach when feasible.

Open-label trial

An open-label trial is one in which treatment identity is known to the relevant parties. This design may be used when single-blind or double-blind conditions are impractical or ethically unsuitable.

Maintenance and breaking of the blind

Breaking the blind for an individual participant may be necessary when knowledge of treatment assignment is essential for that participant’s medical care. Intentional or unintentional unblinding should be reported and explained at the end of the trial. The procedure and timing for revealing treatment assignments should be documented.

Related terminology

  • Bias
  • Protocol
  • Randomization

References

  1. International Council for Harmonisation. 2026. ICH E6(R3): Consolidated Guideline for Good Clinical Practice. Available at: https://database.ich.org/sites/default/files/ICH%20E6%28R3%29_Step4_FinalConsolidatedGuideline_2026_0616_.pdf ↩︎
  2. International Council for Harmonisation. 1998. ICH E9: Statistical Principles for Clinical Trials. Available at: https://database.ich.org/sites/default/files/E9_Guideline.pdf ↩︎