2.B Single-Blind and Double-Blind Procedures

People behave differently when they know what they're supposed to feel. If a participant knows they got the real painkiller, they may report less pain just because they expect relief — that's a participant expectation effect, part of the broader placebo effect. Researchers have their own version of this problem. If an experimenter knows which group a person is in, they may unconsciously smile more, speak more warmly, or interpret a vague response more favorably for the treatment group. This is experimenter bias, and it happens without anyone intending to cheat. Both problems come from the same source: knowledge of group assignment leaking into behavior.

Blinding controls who has that knowledge. In a single-blind study, participants don't know whether they're getting the real treatment or a placebo, but the experimenter does — this removes participant expectation effects but leaves experimenter bias untouched. In a double-blind study, neither the participant nor the experimenter running the sessions knows who is in which group, usually because someone else holds the key until data collection ends. This blocks both sources of bias at once, which is why double-blind designs are the stronger standard, especially in drug trials.

This topic connects directly to confounding variables and internal validity: unblinding is really a story about a confound sneaking back in. If a trial is unblinded partway through, both forms of bias become possible again for whatever data comes after that point, so later results can no longer be cleanly credited to the treatment alone.