Susan wanted to carry out a study to find out if aerobic exercise caused more weight loss than just walking. She instructed one group of study participants to do aerobic exercise for 20 minutes, 4 days a week. A second group was instructed to walk for 20 minutes a day, 4 days a week. Group membership was determined by a coin toss--heads, aerobic exercise; tails, walking. Susan weighed each participant before her study started and again three months into the study and then compared the average weights of the two groups. Susan's study is an example of research.