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The variable the experimenter manipulates.

In experimental research, psychologists create two or more groups that are as similar as possible except for a single change that the psychologist makes from one group to the next. That single element that varies across groups is called the independent variable. In more complex research, the experimenter may include more than one independent variable.

In one experiment dealing with eyewitness testimony and jury decisions, researchers exposed the eyewitnesses to staged crimes and then had them "testify" what they observed. One group of participants saw the staged crime under good lighting conditions; a second group had a less favorable viewing condition, and the third group had only a poor view of the scene. The independent variable was the viewing condition which had three levels, or different variations: good, moderate, and poor visibility. The...