Ruling out Chance as an Explanation (4 of 5)

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The skeptic would argue:

"Since the differences could easily have been produced by chance factors, why should I believe in the effectiveness of this drug? Perhaps the drug is effective, perhaps not. I am not convinced."

Now, for the sake of argument, assume that if there were no true difference between means, the sampling distribution of the difference between means would be as shown below instead of the one shown on the previous page.



Under this assumption, a difference of two or more on the depression scale would occur extremely infrequently, about once in a thousand times. The skeptic would be hard pressed to argue that the antidepressant had no effect. The skeptic's thinking might be as follows on the next page: