Confoundings and Internal Validity

1		An investigator develops the idea that an excess of a particular chemical (RTQX) in the brain during infancy produces permanent mental retardation. To gather evidence for this notion, he uses 30 infant rats and their mothers in an experiment. The rats (infant and mother) are assigned to two groups of 15 infant-mother combinations on a random basis. In the C Group, the infant rats are nursed by the mother rats. In the E Group, infant rats are kept separated from their mothers and fed by bottle. The chemical RTQX is mixed with the milk in the nursing bottle. Tests of mental development are made on both groups at various points in time, even far beyond the nursing period. At every point of the testing the infant rats in the E Group are found to be inferior to those in the C Group. The experimenter concludes that the chemical RTQX causes mental retardation. Do you agree?
2		A psychologist, who also happens to be a musician, decides to test the hypothesis she has always held about the difference between musicians and non-musicians. Specifically, she believes that musicians are born with a "better ear" for music than non-musicians. For instance, she argues that musicians are better able to detect differences in musical sounds than non-musicians. She selects two groups, one made up of musicians and one made up of non-musicians. Musicians are selected from members of the local symphony orchestra; non-musicians are selected from members of a local book club (all of whom have never seriously played a musical instrument). As a test of her hypothesis, she gives both groups a task requiring difficult discriminations between similar musical tones. Her results show that the performance of the musicians is vastly superior to that of the non-musicians. She concludes that her hypothesis that musicians naturally have a better ear for music is strongly supported. Do you agree?