A pharmaceutical company wanted to test a new antistress medication -- Stress-Ex. To do so, they placed an advertisement in the local business magazine asking for highly stressed business executives to take part in a clinical study. Forty such individuals signed up for the study. The participants were asked to rate their level of stress, and then they were placed in a room over a three-day period and given Stress-Ex at regular intervals. At the conclusion of the study, participants were once again asked to rate their level of stress. Thirty of the participants rated their stress level lower at the end of the study. On the basis of this study, Stress-Ex was deemed a success. The evidence cited fails to establish the conclusion because ['there is no evidence that using greater quantities of Stress-Ex would reduce stress', 'the study did not clearly determine the long-term effects of Stress-Ex', "some of the participants' stress levels did not decrease during the study", 'there is no way to know whether stress levels would have gone down without Stress-Ex']