A doctor is studying the effectiveness of a new anti-aging cream. He posts an ad for subjects at two local colleges. He assigns 95 students from University A to the treatment group (daily applications of the experimental anti-aging cream ) and 100 students from Community College ( application) to the control group. After 6 months of the study, Dr. Foster gives all of the students in each group a skin test. The treatment group scores an average of 20 points higher than the control group (a significant difference ). Based on these findings he declares that the anti aging cream increases your skin's youthful glow. Name one way to improve this doctor's experiment .