About the “What researchers mean by...” series

This research term explanation first appeared in a regular column called “What researchers mean by…” that ran in the Institute for Work & Health’s newsletter At Work for over 10 years (2005-2017). The column covered over 35 common research terms used in the health and social sciences. The complete collection of defined terms is available online or in a guide that can be downloaded from the website.

Published: August 2011

If you’ve done a taste test and selected ‘Cola X’ over ‘Cola Y,’ then you’ve already experienced what scientists call “blinding.”

Blinding, in research, refers to a practice where study participants are prevented from knowing certain information that may somehow influence them—thereby tainting the results. Coke versus Pepsi taste trials are conducted in this way: Participants are, literally, blindfolded as they sample the two colas and indicate their preference.

Blinding (also called masking) is typically used in randomized controlled trials (RCTs). In RCTs, people are randomly assigned to two (or more) groups. One group receives the intervention, such as a new treatment, while the control group receives nothing, usual care or a placebo—a fake treatment, an inactive substance like sugar, distilled water or saline solution— when the treatment is a new drug. The researchers then study what happens to each group. Any differences in outcome can then be linked to the intervention, not to the participants’ knowledge of whether they were receiving a new treatment or their usual care.

To ensure to the highest degree possible that the intervention is responsible for any noted differences between the two groups, people involved in gathering or analyzing the data might also be blinded to knowing who is being given the treatment and who is not. This blinding can include clinicians, data collectors, outcome assessors and data analysts. However, certain groups sometimes cannot be blinded, such as surgeons or psychologists who provide active intervention.

Why blinding is necessary

Blinding of one or more parties is done to prevent observer bias. This refers to the fact that most (if not all) researchers will have some expectations regarding the effectiveness of an intervention. Blinding of observers provides a strategy to minimize this form of bias. For example, a clinician who has established expertise in a certain procedure may believe that his or her approach is superior. If involved in a trial to explore this procedure, the clinician may tend to treat patients assigned to his or her procedure differently than patients assigned to the competing intervention.

Blinding is also done to address or control for the placebo effect, a phenomenon in which a simulated (and ineffective) treatment can sometimes improve a patient’s condition, simply because the person has the expectation that it will be beneficial. Expectation is key in the placebo effect.

Landmark study: an example of blinding of patient

In 2002, a study published in the New England Journal of Medicine reported on a controlled trial of arthroscopic surgery for osteoarthritis of the knee. Arthroscopic surgery is the most commonly performed type of orthopedic surgery. In this study by Moseley et al., patients with osteoarthritis—defined as a group of mechanical abnormalities involving the degradation of joints—were divided into two groups: one receiving corrective surgery (arthroscopic debridement), and the other receiving fake or sham surgery.

The patients were blinded in the sense that they did not know whether they were receiving the real or sham surgery. The results were quite surprising: Both groups of patients improved equally well regardless of whether or not they received the real surgery. This is an excellent example of the placebo effect and the need for blinding, since it implies that belief of recovery alone can have an effect, even on a mechanical knee problem.

Source: At Work, Issue 65, Summer 2011: Institute for Work & Health, Toronto