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

Experimental Controls

For every confounded variable, there is a potential control. In one sense of the term, to control a variable is precisely to remove it as a confounded variable. This is called methodological control; it means a variable is "ruled out" as a confounded variable by the logic of the experiment.

What does the word "control" mean when used in the context of experimental design?

Most beginning students take the word "control" literally; they visualize a laboratory scientist holding some variable such as temperature steady. But physical control over variables is not always necessary. Methodological control can be achieved, sometimes, by merely measuring a variable. Then one can demonstrate there is no difference between two groups on that variable, or the difference is not in a direction that would cause trouble when interpreting the data.

How could the student have eliminated volume as a confounded variable in his research?

On some occasions, an experimenter controls a variable by holding it steady. In the example on the previous page, of a student testing how different types of music affected studying, if he had realized the volume of his recordings might be a confounded variable, he could have controlled this variable by making sure the volume was exactly equal on the two tracks.

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