A certain bon vivant, wishing to ascertain the cause of his frequent hangovers, conducted the following experiment. On the first night, he drank nothing but whiskey and water; on the second night, he drank vodka and water; on the third night, he drank gin and water; and on the fourth night, he drank rum and water. On each of the following mornings he had a hangover, and he concluded that it was the common factor, the water, that made him ill.

(a) This conclusion is obviously unwarranted, but can you state what principles of sound experimental design are violated?

(b) Give a less obvious example of an experiment having the same shortcoming.

(c) Suppose that our friend has modified his experiment so that each of the 4 alcoholic beverages was used both with and without water, so that the experiment lasted 8 nights. Could the results of this enlarged experiment serve to support or refute the hypothesis that water was the cause of the hangovers? Explain.