Suppose a study finds that children who regularly eat breakfast perform better at school than those who do not. The study concludes that eating breakfast increases performance at school. Given your background knowledge, which of the following should make you most suspicious of drawing this conclusion from the data?

a. In the study, the number of subjects who regularly ate breakfast was higher than the number of subjects who did not.

b. There was no group of subjects in the study who never ate breakfast at all.

c. The study relied on subjects to self-report their eating habits.

d. The subjects were all Norwegian children.

e. There was no attempt to control for socioeconomic status or parenting differences.