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KINDS OF CONFOUNDS: THREATS TO VALIDITY

It’s pointless to ask questions about external validity until you establish internal validity. In a series of influential publications, Donald Campbell and his colleagues identified the threats to internal validity of experiments (see Campbell 1957, 1979; Campbell and Stanley 1966; Cook and Campbell 1979). Here are seven of the most important confounds:

History

The history confound refers to any independent variable, other than the treatment, that (1) occurs between the pretest and the posttest in an experiment and (2) affects the experimental groups differently. Suppose you are doing a laboratory experiment with two groups (experimental and control) and there is a power failure in the building. So long as the lights go out for both groups, there is no problem. But if the lights go out for one group and not the other, it’s difficult to tell whether it was the treatment or the power failure that causes changes in the dependent variable.

In a laboratory experiment, history is controlled by isolating participants as much as possible from outside influences. When we do experiments outside the laboratory, it is almost impossible to keep new independent variables from creeping in and confounding things.

Here’s an example of an experiment outside the lab. Suppose you run an experiment to test whether monetary incentives help third graders do better in arithmetic. Kids in the treatment classes get a penny for each right answer on their tests; kids in the control classes get nothing. Now, right in the middle of the school term, while you’re running this experiment, the Governor’s Task Force on Education issues its long-awaited report, with a recommendation that arithmetic skills be emphasized during the early school years. Furthermore, it says, teachers whose classes make exceptional progress in this area should be rewarded with 10% salary bonuses.

The governor accepts the recommendation and announces a request for a special legislative appropriation. Elementary teachers all over the state start paying extra attention to arithmetic skills. Even supposing that the students in the treatment classes do better than those in the control classes in your experiment, we can’t tell if the magnitude of the difference would not have been greater had this historical confound not occurred. That’s just the breaks of experimenting in real life without being able to control everything.

 
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