Epidemiological studies of lung cancer--influence of smoking habits

It is important to evaluate epidemiological studies of lung cancer even when the smoking habits of the investigated groups are unknown. The increased risk for lung cancer in heavy smokers is known from other studies. Assuming a reference population consisting of 30% non-smokers, 25% ex-smokers, 40% moderate smokers and 5% non-smokers, it can be shown that a more than doubled incidence cannot be explained by differences in smoking habits.