What is a "control" in an experiment?

Experimental controls are mechanisms in science that eliminate extraneous factors that might otherwise affect the results of an experiment. By creating a second set that is unaffected by the phenomenon being measured, scientists are able to isolate the phenomenon by contrasting the test group with the control group.

A common example of an experimental control is the "control group" used in double-blind medical testing. In the simplest version of this process, one group is given an experimental medicine and another group is given an inactive placebo that is identical in appearance. The results from both groups are then compared to see whether the medicine has any noticeable affect beyond the placebo effect.