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An intervening variable is a hypothetical internal state that is used to explain relationships between observed variables, such as independent and dependent variables, in empirical research.
The term “intervening variable” was first used by behavioral psychologist Edward C. Tolman in 1938.
Relation to operational definitions
An intervening variable facilitates a better understanding of the relationship between the independent and dependent variables when the variables appear to not have a definite connection. They are studied by means of operational definitions and have no existence apart. For example, an independent variable in a study on latent learning in rats is the number of practice trials received. Each rat receives an increasing number of trials, as one trial is given per day. The dependent variable is the number of wrong turns (errors) the rats make on a trial. As time, and number of practice trials, increases, the number of errors decreases. Theoretically, an internal state of “learning” intervened between the independent and dependent variables. It was this state that caused the errors to decrease, not the practice trials.
Other examples of intervening variables include: motivation, intelligence, intention, and expectation.
Intervening variables and circular reasoning
As explanations of behavior, intervening variables can lead to an error in logic called “circular reasoning.” To avoid circular reasoning, two or more operational definitions of the same internal state must be present, and they must be correlated.
- Shaughnessy J.J., Zechmeister E. & Zechmeister J. (2006). Research Methods in Psychology (7th ed., pp.51–52). New York: McGraw Hill.
- Tolman, E. C. (1938). The Determiners of Behavior at a Choice Point. Psychological Review, 45, 1-41.
- Tolman, E. C. and C. H. Honzik. (1930). Degrees of hunger, reward and nonreward, and maze learning in rats. University of California Publications in Psychology, 4, 241-275.
- Online text of "The Determiner of Behavior at a Choice Point"
- Kenneth MacCorquodale and Paul E. Meehl (1948) ON A DISTINCTION BETWEEN HYPOTHETICAL CONSTRUCTS AND INTERVENING VARIABLES Classics in the History of Psychology, retr. 22 Aug 2011.
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