A teacher wants to know if burning a scented candle affects students’ performance on pop quizzes. In the experiment, there will be a treatment group and a control group each taking their quizzes in a room that either does or does not have a scented candle burning.

Randomly assigning students to the treatment and control groups helps make the groups roughly equivalent in characteristics that could affect quiz performance, such as preparedness, anxiety, and study skills. This makes it more reasonable to attribute a difference in scores to the scented candle rather than to preexisting differences between the groups. Matching room conditions, as in choice D, is also important, but it is controlled by keeping the testing environments similar, not by assigning students randomly.

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