Do diet sodas contribute to obesity? Researchers recruited 1,200 randomly selected college students at state universities in Texas to participate in a four-year study looking at the relationship between diet soda and obesity. At the start of the study, students were weighed and interviewed. After interviews with the students, participants were classified as either regular soda drinkers, diet soda drinkers, or non-soda drinkers. Four years after the interview, the students were contacted again and reweighed. The weight gains were then compared between the diet soda drinkers and non-soda drinkers. This study appears to be: