Blinding / Masking
What a definition is not
A definition is SPEQ’s plain-language decode of how a term is used in practice, cited to the documents that define it. It is a practitioner reference, not legal or regulatory advice, it does not replace the definition in the source, and where a regulator’s wording differs the regulator’s wording governs.
A procedure in which one or more parties to a trial — subjects, investigators, or outcome assessors — are kept unaware of treatment assignment, to reduce the risk of conscious or unconscious bias in trial conduct, data collection, and outcome assessment. Trials may be open-label, single-blind, or double-blind, chosen according to the endpoint and feasibility.
Blinding exists to protect against several distinct sources of bias: differential expectation effects on reported symptoms, differential care or dropout driven by knowing the assignment, and biased assessment of subjective endpoints by investigators or raters who know which arm a subject is in.
Double-blind designs mask both subject and investigator/assessor and are generally preferred where feasible; single-blind masks only the subject. Where full blinding is not practical — for example in surgical or device trials — an independent blinded outcome assessor is often used to preserve assessment objectivity even though the subject and treating clinician are unblinded.
Emergency unblinding of an individual subject, done for their safety, must follow a pre-defined procedure and is documented; it is typically treated as a protocol deviation and reviewed to confirm it was necessary and properly recorded.
- —Double-blind designs mask both subject and investigator/assessor; single-blind masks only the subject.
- —Emergency unblinding for subject safety must be documented and is typically treated as a protocol deviation.
- —Where full blinding is not feasible, independent blinded outcome assessors are often used instead.
- —Unplanned loss of blinding during a trial can undermine the validity of the primary analysis and must be assessed.
ICH E9 Statistical Principles for Clinical Trials; ICH E10 Choice of Control Group and Related Issues in Clinical Trials
Frequently asked questions
What is Blinding / Masking?
A procedure in which one or more parties to a trial — subjects, investigators, or outcome assessors — are kept unaware of treatment assignment, to reduce the risk of conscious or unconscious bias in trial conduct, data collection, and outcome assessment. Trials may be open-label, single-blind, or double-blind, chosen according to the endpoint and feasibility.
Which regulations cover Blinding / Masking?
ICH E9 Statistical Principles for Clinical Trials; ICH E10 Choice of Control Group and Related Issues in Clinical Trials