All available data from the four study waves of FT12 were used. In total, some data were available for 5,293 twin individuals (2,625 or 49.6% females). The number of observations varied for each study wave and variable, ranging between 1,869 and 4,709 (Table 1). The smaller number for age 12 alcohol and smoking variables was because that information was available only from an intensively studied sub-sample. Namely, nested within the full population-representative FT12 study is a more intensively studied sub-sample of 2,070 twin individuals who were asked to complete an in-school questionnaire at age 12, including items on alcohol use and smoking (Kaprio et al., 2002).
