Longitudinal twin data suggests a high correlation between these variables over time, which supported using either value in our analyses. For studies that collected CPD as ranges, the mid-point of the interval was used as the data point; for example, individuals who reported the CPD category 0–4 were assigned a CPD value of 2. SC contrasted individuals who had quit smoking at interview (former smokers) with those who were current smokers. As relapse to smoking is highest within the first year after quitting,23 we tried to reduce misclassification by excluding smokers who quit within 1 year of interview within studies with available data. Table 1 presents distributions of smoking phenotypes across participating studies.
