Dr. DeVries is interested in measuring how practice in problem solving affects this ability. The population of interest is high school seniors, and the sample consists of students who attend an exclusive college preparatory school. Half of the sample receives practice in solving particular types of problems, while the other half does not. Both groups take the same problem-solving test. Which of the following might best explain why Dr. DeVries may not be able to generalize his findings?
A) He is not taking into account how the scores on the test might deviate from one another.
B) His data may be positively skewed.
C) He should have conducted his experiment in a lab instead of a natural setting.
D) His sample is not representative of the population.