Scientist: To study the comparative effectiveness of two experimental medications for athlete' s foot, a representative sample of people with athlete' s foot were randomly assigned to one of two groups  One group received only medication M, and the other received only medication N  The only people whose athlete' s foot was cured had been given medication M
Reporter: This means, then, that if anyone in the study had athlete' s foot that was not cured, that person did not receive medication M
The reporter concludes from evidence showing only that M can cure athlete's foot that M always cures athlete's foot.