Martha believes that a relaxation technique involving visualization will help people with mild insomnia fall asleep faster. She randomly selects a sample of 20 participants from a group of mild insomnia patients and randomly assigns 10 to receive visualization therapy. The other 10 participants receive no treatment. Each participant is then measured to see how long (in minutes) it takes him or her to fall asleep. Martha's data are below. The numbers represent the number of minutes each participant took to fall asleep.