29 29 0.66 points A researcher hypothesized that pop quizzes improve college students' final course grades. One CCJ4700 professor always gives weekly pop quizzes, and another CCJ4700 professor never gives pop quizzes. Last semester, the researcher compared the average final grades of students in the pop quiz class with those of students in the no quiz class. He found that students in the pop quiz class received higher final course grades. If this study included _, it would strengthen the researcher's conclusion that there is no alternative explanation for the observed association. Random assignment A comparison group Replication An independent variable