A researcher was interested in finding out whether children's ability to exhibit self-control when offered unhealthy food changed with age. He asked parents of three-year-old, six-year-old, and nine-year-old children to bring their children into the laboratory, where they were allowed to eat as much candy, cookies, and cake as they wanted. The researcher then compared the amount of food eaten by children of the three different ages. This is an example of:_________.