We defined this as part of our definition for ‘double blinded,’ but here it is again!
For this explanation, let’s imagine a checkout counter at a store. You are buying something. I am behind the counter as a cashier. In this case, let’s designate you as the participant and me as the experimenter. Let’s say that this is a very bad store where there are only two types of shirts being sold: t-shirts and tank tops.
In a single-blinded study, you as the participant would not know whether you are buying a t-shirt or a tank top. All you would know is that you’re holding a mysterious brown packet that is a type of shirt, and that you’ve now brought it to the counter to check out. As the experimenter, I would know whether you have a t-shirt or a tank top. While you are ‘blinded’ to which category you’re in, I am not. This is a single-blind study.
In a double-blind study, neither you nor I know what’s in the packet. All I know is that you have either a shirt or a tank top and have come to the counter to check out. All you know is you’ve bought some sort of shirt. In this case, we are both blinded to the category you’ve fallen into- so this is a double-blind study.
In the case of a double-blind study, the actual categories are stored in a database somewhere or blacked out of the data files before scientists analyze them. Single or double-blinded studies help make sure that the results aren’t biased- either by patient behaviour or by the experimental analysis, or both.