37. The article “Smoking, Smoking Cessation, and Risk for Type 2 Diabetes Mellitus” published in the Annals of Internal Medicine (January 2010) reported on a study that followed 10,892 middle-aged adults over a nine-year period. At the start of the study, none of the subjects had diabetes. Roughly 45% of the subjects were smokers. The study found that compared to those who never smoked, subjects who quit smoking had an increased risk of diabetes.
37.
(a) This is an observational study—a prospective study. It takes a group of people, both smokers and nonsmokers, and observes them over a nine-year period. The response variable is whether or not the subject develops diabetes. The purpose of the study is to describe the response variable (diabetic/not diabetic) for those who were smokers versus nonsmokers at the start of the study, as well as those who were smokers and later quit smoking.
(b) You cannot conclude that quitting smoking causes diabetes. Most people who quit smoking also gain weight. Weight gain is also associated with diabetes. So, it would be impossible to tell whether the diabetes was caused by the cessation of smoking or the gain in weight.