Placebo vs Blinding

A placebo is an inactive version of the treatment: a sugar pill, a saline injection, a sham procedure. It is given rather than withheld, so the placebo group is receiving a treatment in the design, one that does nothing on its own. The placebo effect has a definition worth stating exactly, because it is a difference between two averages: it is the average response to a placebo minus the average response to no treatment at all.

Three arms make that visible. Suppose 150 chronic pain patients are randomly assigned in equal groups, 50 taking the drug, 50 taking an identical inactive pill, 50 taking nothing. Mean pain reduction comes out 3.6 points for the drug, 2.1 for the placebo, and 0.9 for no treatment. The placebo effect is $2.1 - 0.9 = 1.2$ points. The drug's own effect is $3.6 - 2.1 = 1.5$ points. Had the trial compared the drug against no treatment, it would have credited the active ingredient with $3.6 - 0.9 = 2.7$ points, nearly double what it did.

The sentence to correct is "the control group gets a placebo." Sometimes. A control group is defined by supplying the baseline, not by what it swallows, and plenty of sound experiments have one with no placebo in sight. The error that runs the other way is "we used a placebo, so the study was double-blind." A placebo makes it possible to keep subjects unaware of their assignment; it does not do the keeping. If the labels differ, or the nurse handing out the bottles knows which is which, somebody is not blind.

Placebos are unavailable more often than not. There is no inactive version of a twelve-week exercise program or a smaller class size, and a sham surgery raises an ethical problem a sugar pill does not. They also do nothing for units that cannot form expectations, which is why plant and machine experiments never use them.

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.