50. The effect of an outside variable can be surprising when individuals are divided into groups. In recent years, the mean SAT score of all high school seniors has increased. But the mean SAT score has decreased for students at each level of high school grades (A, B, C, etc.). Explain how grade inflation in high school can account for this pattern. A relationship that holds for each group within a population need not hold (and may even be in the opposite direction!) for the population as a whole.