Slide 1
Knowing the group can influence behavior
Blinding keeps selected people unaware of assigned treatment groups during a trial. It aims to reduce effects of expectations on care or measurement.
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Participants, clinicians and outcome assessors can each influence different parts of a study. A report should say who was unaware of the assignments rather than relying only on an ambiguous label such as double-blind.
Slide 2
Expectations can alter reported outcomes
A participant’s expectations can affect how symptoms are experienced or reported. Keeping assignments unknown can make that influence more similar across groups.
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Blinding does not imply that symptoms are imaginary. Subjective outcomes are real experiences, but measuring them can be especially sensitive to what participants believe about the treatment they received.
Slide 3
Assessors can be blinded separately
A person evaluating an outcome may remain unaware of treatment assignment even when participants know it. This can reduce assessment bias.
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For example, an assessor reviewing a recorded examination need not necessarily know which intervention was used. Separating treatment delivery from evaluation can preserve a useful safeguard in an otherwise open study.
Slide 4
Some interventions are difficult to disguise
A surgical procedure or exercise program may be recognizable to participants. Blinding every role is not always practical or ethical.
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Researchers can still standardize measurements and specify outcomes in advance. Lack of blinding must be evaluated in relation to the outcome and methods, rather than automatically treating every unblinded study as worthless.
Slide 5
Blinding and concealment protect different stages
Allocation concealment protects enrollment before assignment. Blinding addresses what people know after assignment while the study is being conducted.
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An apparently blinded treatment can still have been assigned through a flawed recruitment process. Checking both stages prevents one reassuring feature from hiding a separate weakness in the design.
Sources
- How Clinical Trials Work
NHLBI, NIH
- Clinical Trials
NIH Office of Behavioral and Social Sciences Research
- Assessing risk of bias in a randomized trial
Cochrane Handbook, chapter 8