흡연은 현재 담배를 피우고 있는 현재 흡연군, 담배를 피우다가 끊은 과거 흡연군, 전혀 담배를 피우지 않는 비흡연군으로 정의하였다.
Smoking was defined as the current smoker group who currently smoked, the past smoker group who quit smoking and the non-smoker group who did not smoke at all.