Kanchi is interested in studying how water aerobics improves attitudes toward exercise over time. She recruits a group of participants who have signed up for a water aerobics course and measures their attitudes about exercise. She randomly assigns half of the participants to begin a weekly water aerobics class immediately and assigns the other half of participants to start taking the weekly aerobics class the following month. After all the participants have taken two months of aerobics, Kanchi measures their attitudes about exercise again. What design strategy is Kanchi using? nonequivalent control group posttest-only design interrupted time-series design wait-list design matched-groups design