Blinding

Blinding is a design feature that keeps knowledge of the treatment assignment away from someone in the experiment. In a single-blind study, either the subjects do not know which treatment they are getting, or the people who interact with them and record the outcome do not. In a double-blind study neither side knows. Two different things are being protected: the response itself, which a subject's expectations can move, and the measurement of the response, which the recorder's expectations can move.

The second one is easy to underrate. Suppose 80 patients rate knee pain from 0 to 10 after an unmarked gel, 40 on the new formula and 40 on an identical inactive one. If the nurse recording the ratings knows who got what and records each treated patient half a point lower than the patient reported, the treated group's mean falls by exactly 0.5. No single rating looks wrong, and a half point on a 0 to 10 scale can be the whole of the effect the trial was built to find.

The misreading is a vocabulary slip: "neither the treatment group nor the control group knew which one they were in, so the study was double-blind." Both groups being kept in the dark is one blind, not two. The second blind covers a different kind of person, the one delivering the treatment or scoring the outcome.

Some treatments cannot be hidden from the subject. Nobody undergoing surgery or a twelve-week exercise program is unaware of it. The response can still be scored by someone who does not know the assignment, so blinding the assessor is the usual fallback, and this is also why a placebo matters: an inactive lookalike is what makes blinding the subject possible at all.

Blinding balances nothing. It cannot stand in for random assignment and it does not remove a confounding variable; what it removes is expectation. Single-blind and double-blind are named in topic 1.13 Experimental Design.