with multiple response categories, ranging from “I have never smoked, not even one puff” to “I smoke at least once a day” and recode responses to either no (non-smoker) to yes (smoker) (9,10). There is also some variation in how to assess cigarettes per day: some studies collect the average number of cigarettes smoked per day, while others collect the maximum number of cigarettes per day (11). Nicotine dependence is most often assessed using the Fagerstrom Test for Nicotine Dependence (12). Smoking cessation is assessed in a variety of ways, though the most common seems to be through self-reports of abstinence (e.g. 7-day point prevalence abstinence, 30-day prolonged abstinence, 6-month prolonged abstinence) or by asking about quit attempts. These different stages of cigarette use vary in their heritability, suggesting that different points along smoking trajectories may be influenced by different etiological factors (13). Distinguishing between these phenotypes helps to provide insight into the nature of cigarette use, which may provide guidance for potential interventions and treatments (14).
