A study of more than 50,000 U.S. nurses found that those who drank just one soda or fruit punch a day tended to gain much more weight and had an 80% increased risk of developing diabetes compared to those who drank less than one a month. (The Washington Post, August 25, 2004). "The message is clear. . . . Anyone who cares about their health or the health of their family would not consume these beverages," said Walter Willett of the Harvard School of Public Health, who helped conduct the study. The sugar and beverage industries said that the study was fundamentally flawed. "These allegations are inflammatory. Women who drink a lot of soda may simply have generally unhealthy lifestyles," said Richard Adamson of the American Beverage Association.

a. Do you think that the study described was an observational study or an experiment?

b. Is it reasonable to conclude that drinking soda or fruit punch causes the observed increased risk of diabetes? Why or why not?