Blinding is one of those principles of experimental design whereby the subjects don't know what treatments they're going to receive.
When you randomize an experiment, it is done to reduce bias. However, it's possible to give subtle clues regarding what treatment they're receiving; it’s important that the people don’t know what they're receiving.
Why is this? Because it might be an incentive for them to either stay on the treatment if it's a drug or go off the treatment if they think they're not getting the real drug.
Also, it may be true that people with an agenda might want to bend the results in their favor. They might want to make the results of an experiment seem more positive than they really are. This idea of the experimenter wanting to bend the results in their favor is called the “experimenter effect”.
To counteract both of those two ideas, we implement a strategy called blinding. Only people who are behind the scenes will know who is getting what. No one, either directly involved in the experiment or taking any of the treatments, knows what treatments they're receiving.
Ideally, when you open the pills up, they would look the same on the inside, too. The idea is that no one knows which pill is fake and which one has the tested drug.
The fake drug is usually some kind of a sugar or something that makes the person in the control group feel like they're actually taking something when they’re really not.
A lot of the times, experiments are what we call double-blind. Double-blind experiments means that the subjects don't know what treatment they're receiving, nor does anyone who has any contact with them. This can eliminate bias, due to a subject thinking they know what group they're in. It also reduces the experimenter effect of someone trying to bend the results.
Single-blind experiments, on the other hand, can have subjects blinded, but the researchers are not.
However, the experimenters don't need to who was assigned not to exercise. This is single-blind because the experimenters don't know. The experimenters were blinded, but the subjects were not.
Source: Adapted from Sophia tutorial by Jonathan Osters.