ACT Preparation / Unit 20 ACT 1916 worked example, 60 to 90 seconds
Spotting a Confound
The idea
A confounding variable is spotted in three steps. First, state the comparison the study makes: which groups, and which difference in treatment. Second, look for any other difference between those groups: in who the subjects were, and where, when or how they were treated. Third, ask whether that difference could affect the result; if it could, it is a confound, and the conclusion is weakened.
In practice
Worked example: an invented study compares the test scores of students who chose an online review course with those of students who did not, and finds the online group scored higher. Another difference: students who chose the course may have been more motivated. Motivation could raise scores on its own, so it is a confound. Random assignment to the course would have spread motivation across both groups.
Where people slip
When subjects choose their own treatment, suspect self-selection first.
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