Blinding, also called masking, refers to concealing from participants and/or study teams who are and are not receiving an intervention or placebo in a clinical trial.
Deeper dive
When researchers conduct clinical trials that randomize participants, they have to contend with the possibility that participants will have psychosomatic responses to their treatments, such as expecting that they will get better — and subsequently feeling better — if they know they are receiving a medication that is intended to treat a condition they have. Similarly, the person administering a treatment might convey through body language, verbal language or some other way, even inadvertently, that a participant is receiving the real medication as opposed to the placebo. If participants know who is receiving the actual, pharmacologically active medication and who is receiving the placebo, it could bias the results based on what is known about the placebo effect and nocebo effect and other types of biases.
Therefore, when possible, researchers employ blinding during trials so that participants and/or those administering a medication or treatment do not know which is the real treatment and which is the placebo or control treatment. Those administering the treatments may also be blinded from knowing which participants have a condition and which don’t if the control group is individuals who do not have a condition.
During a single-blind trial, only the participants (or, more rarely, only the researchers/administrators of a medication/treatment) do not know who is in the control group and who is in the experimental group. With double blinding (double masking), both the participants and the researchers are prevented from knowing who is in the control group and who is in the experimental group. (Note: Sometimes “triple blinding” is used if the participants, the researchers and the person administering a treatment do not know who is in the control versus experimental groups, but this same scenario is often covered under double blinding as well.)
Blinding can also refer to removing results or a time period from an analysis if leaving them in could bias the analysis of the data. For example, if a long-term study involving women looks at side effects that could result from use of birth control, and some women in the study become pregnant, their period of gestation may be “blinded” in the analysis since any of the outcomes studied that occur during that time would likely be caused by the pregnancy or some other factor and not from the birth control.
In some single-blind procedures, patients are not receiving a placebo but instead a “sham treatment” that resembles a non-pharmacological intervention being tested. In acupuncture, for example, a sham procedure would involve inserting needles into the control participants but at different body locations than what traditional acupuncture would call for.