This is the 2007 version. Click here for the 2017 chapter 01 table of contents.

Those Confounded Variables!

In a between-subjects comparison, where different groups receive different manipulations, the experimenter tries to create a situation in which there is only one consistent difference between experimental and control groups. The ideal is to isolate the effects of a single causal variable. (By the way, that word is causal not casual... "causal" is pronounced CAW-zal and means "suspected of being a cause.")

The reason a researcher manipulates a particular independent variable is that this variable is suspected of causing some effect. The effect, if any, is expected to show up in measurements of the dependent variables.

What is the ideal experiment?

Ideally, an experimenter wants the manipulation of the independent variable to be the only difference between groups. But this is an ideal seldom attained. There are often other, unintended differences between experimental and control groups. They are called "confounded" variables.

What are confounded variables? What does "confounded" mean?

The word "confounded" means "confused" or "mingled together." Unintended differences are "mingled in" with the difference an experimenter intends to create between groups. They are unwanted because they make it impossible to interpret the research.

One psychology major did not appreciate the importance of confounded variables until he did a research project for an experimental psychology course. His hypothesis was that the type of background music would affect performance on complex tasks. Subjects tried to solve anagrams (word puzzles) while music played in the background. Group #1 heard the soothing strains of an old Allman Brothers song, "Dedicated to Elizabeth Reed." Group #2 heard an unbelievably jarring and repetitious song: "Brainwash" by the punk band Flipper. But the psychology student recorded the Allman Brothers song at a higher volume, and during his experiment he noticed the subjects were doing worse during the Allman Brothers song. As he put it:

How did a confounded variable foul up the student's experiment on music and puzzle-solving?

Suddenly I realized my experiment was ruined. I could not tell if they were doing worse because of the difference in music or the difference in volume. Then I really understood the concept of confounded variables for the first time. [Author's files]

In comparisons between groups, a confounded variable is any difference between groups other than the one an experimenter deliberately creates. A confounded variable frustrates any attempt to pin down a cause-effect relationship, because it provides an alternative explanation for the results. If an "effect" is observed, nobody can tell whether it is due to the manipulation of the independent variable or the presence of the confounding variable. Therefore confounding (or "confounded") variables must be eliminated whenever possible.

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Copyright © 2007-2011 Russ Dewey