A study at Yale Medical School investigated treatments for chronic cocaine abusers. Of the 72 subjects, all cocaine users who wanted to quit, 24 were assigned at random to each of three medications: (a) desipramine, (b) lithium, (c) a placebo. After six weeks the subjects reported whether they had stayed off cocaine. The explanatory variable in the Yale study is:

The explanatory variable in the Yale study is which medication a subject was given.

Explanation:

The response variable in a study is the focus of the question of that study. In this particular case, what the study aims to know is whether a subject was able to stay off cocaine. The explanatory variable is the one that may or may not influence the response variable. In this case, what might influence the subjects is the medication given to him/her.

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