Single, double and triple blinding
Blinding keeps people in a study unaware of which treatment each participant was allocated, and single, double and triple blind name which groups are kept unaware.
Remember
Count the groups kept unaware: only the patient for single blind, the patient and the clinician or assessor for double blind, and the data analyst (the statistician who analyses the data) as well for triple blind.
Memory aid
Count who is kept in the dark
Count the groups kept unaware: the patient for single, the patient and the clinician or assessor for double, and the analyst as well for triple.
Blinding, also called masking, hides the treatment allocation, meaning which treatment each participant was given, from the people in a study. This reduces the chance that their expectations distort the results.
The labels count the groups kept unaware. In a single-blind study only the patient is unaware, so the clinician or assessor (the person assessing the outcome) could still be influenced by knowing the treatment. In a double-blind study the clinician or assessor is unaware as well as the patient. Triple blind adds the data analyst (the statistician who analyses the data) as a further blinded group. It is rarely used, and some authors regard it as part of double blinding. Because authors use these labels differently, a good report states exactly who was blinded.
Blinding guards against the placebo effect, observer bias and reporting bias. The placebo effect is an improvement produced by expectation alone. Observer bias means that an assessor's expectations alter how outcomes are judged. In the context of blinding, reporting bias arises when patients or doctors report results differently because they know the treatment.
Do not confuse blinding with randomisation, which decides by chance who joins each group before treatment starts. Blinding happens after allocation. Allocation concealment is also separate: it hides the next allocation from the person recruiting participants.
Where blinding is impossible, as in surgery, researchers use a sham procedure, a pretend operation, and blinded outcome assessment. The operating surgeon cannot be blinded, yet the participant, the assessor and the analyst can still be.
Key facts
- Single blind
- only the patient is unaware of the allocation.
- Double blind
- the patient and the clinician or assessor are both unaware of the allocation.
- Triple blind
- the data analyst is blinded as well, which is rarely used.
- Open label
- there is no blinding of the participants or the researchers, so they know the allocation.
- What blinding reduces
- the placebo effect, observer bias and reporting bias.
- When blinding is impossible
- a sham procedure and blinded outcome assessment recover as much of the protection against bias as they can.
Exam traps
- Random allocation to groups is randomisation, a separate feature, so on its own it does not make a study double blind, because it does not hide the allocation from anyone once the treatment is given.
- Giving both groups a placebo does not establish who knows the allocation, because a placebo is a type of control treatment and not a form of blinding.
- One person knowing the allocation, such as an operating surgeon, does not make a study open label, because open label describes a study with no blinding, where both the participants and the researchers know the allocation.
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