Placebo

A placebo controls for the placebo effect, the tendency of people to respond simply because they believe they are being treated. A sugar pill, saline injection, or sham procedure looks and feels like the real thing but does nothing on its own. For example, in a drug trial the control group takes an identical-looking sugar pill so both groups share the same expectations. Any difference in outcomes between the groups can then be credited to the drug's active ingredient rather than to belief.