A researcher studies alcohol's effect on reaction time and finds no difference between people who consumed 1 versus 2 beverages. She included 40 participants randomly selected from the dining hall on campus, with a range in age from 18 to 38 years and a range in weight from 100 to 325 pounds. She decides to rerun her study with 30 women from 18 to 22 years of age and who are all of normal body weight and finds there is a statistical difference in reaction times between those who consumed 1 versus 2 beverages. Why might her results have changed?