Researchers conducted a case-control study to determine if there is an association between lung cancer and a spouse who smokes cigarettes indoors. The study restricted subjects who had ever smoked cigarettes from entering the study. All subjects were married. There were 100 lung cancer patients and 120 controls. Among the 120 controls, 90 reported that their spouse did not smoke cigarettes indoors. Based on the study data, an odds ratio of 2.1 was observed for the association between lung cancer and a spouse who smokes cigarettes indoors. Why did the researchers exclude subjects who had ever smoked cigarettes from entering the study?