Simply knowing which group they are in can change how participants feel, even before a treatment begins.
A placebo lets both groups believe they might be getting the real treatment, so expectations are similar and the real medication is the only difference between groups.
If the treatment group improves more than the placebo group, the extra improvement suggests the treatment works; equal improvement points to the placebo effect.
In a single-blind study participants don't know whether they are in the experimental or control group, but the researcher's expectations can still cause experimenter bias.
Double-blind studies keep both participants and the researchers interacting with them unaware of group assignments, controlling the placebo effect and reducing experimenter bias.
Double-blind studies are considered the gold standard for testing many new treatments.
Key terms
placebo
A fake treatment that looks like the real one but contains no active ingredient.
placebo effect
The tendency for people to feel better simply because they believe they're receiving treatment.
single-blind study
A study in which participants don't know whether they are in the experimental group or the control group.
experimenter bias
When a researcher's expectations unintentionally influence a study, through interactions with participants or interpretation of results.
double-blind study
A study in which neither participants nor the researchers interacting with them know who is in which group.
Watch out
Single-blind = only participants are blind; double-blind = participants and researchers are blind. Only double-blind reduces experimenter bias.