A professor conducts an experiment that investigates the effects of a new statistics CD on the student understanding. His one statistics class is used for the sample. He divides the roster into two halves and uses each half as a group. One group like receives the CD and the other does not receive the CD. Each group consists of 10 students. Observations of students classroom engagement are rated by the professor, according to an engagement scale. How could this study be improved? (Select all that apply)
A. Use an instrument that measures student understanding.
B. Repeat the study again, where a larger sample size can be derived.
C. Randomly assign the students to the two different groups.
D. Ask another professor to make and rate observations, as well.