How do I "control" a variable?

• Compare at least two conditions

An important feature of experimental designs is that the researcher compares two (or more) conditions or groups. In one condition, a "treatment" is present in the situation (called the "treatment" condition), and in another condition, the treatment is absent (the "control" or "comparison" condition). The Pennebaker and Francis (1996) experiment on college students' adjustment was an experiment with two conditions. The "emotional writing" condition was the treatment condition, and the "superficial writing" condition was the comparison condition. More information about their experiment is provided in the Research Example portion of this Student as Researcher CD-ROM.

An additional example might help to illustrate what we mean by experimental control. This example comes from a different area of research in psychology: How people respond when someone hurts or angers them. A victim's natural response to an offense is to want revenge. What can be done to reduce retaliation and aggression following an interpersonal injury? We might ask whether an offender's apology following a harmful action decreases the likelihood that the victim will want revenge. Here's one possible sequence of events in this situation:

After reviewing research literature on this topic, we may hypothesize that the presence of an apology, compared to no apology, decreases the likelihood of victims' desire for revenge.

To test this hypothesis, we may create two hypothetical scenarios ("vignettes") to describe an offense. We could control whether the offender apologizes or does not apologize in the scenario. The two scenarios would be identical, except for the presence or absence of an apology. This would be the operational definition of apology in this experiment.