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31%) among those who were lost to follow-up than those who were followed for the entire study (8.17%). Data collection This study utilised a self-administered questionnaire to collect data, which were taken by the students themselves in the classroom. In 2008, the data collection procedures included three steps. First, we contacted the school Telomerase administrators and teachers. The second step was to obtain consent from the students and their parents. All students who provided parental and individual informed consent were included in the study. Finally, trained interviewers collected data from the classes after explaining the purpose of the study and reiterating the ethical issues associated with this research. Measurement of variables The independent variable was smoking initiation that was measured by asking students who had never smoked at 2008, of the question- ��Have you ever smoked or made attempt to smoke cigarette����and with a binary answer (yes/no) at the 2009 follow-up. All independent variables were collected from the baseline questionnaire at the entry of study in 2008. The main independent variable, HSR, was based on students�� self-reported information on whether the definite inhibition of smoking at home or not was stipulated and implemented. Parental smoking behaviour was also reported by students in a similar manner with the question ��Did your father/mother smoke in the preceding year?�� Furthermore, we collected information about the person who made the decision about HSR in a multiple selection question. Then we organised the data into four categories as follows: father only, mother only, both father and mother and collective decision made by parents and other members (such as grandparents). Additional explanatory variables included in the following regression model, which were also collected from the 2008 baseline questionnaire, included sex (boy/girl), highest parental education (junior high school/senior high school/college or university), having a best friend who smoked (yes/no), and intention to smoke in the future (yes/no). The latter two variables were measured by asking the two questions: ��Did your best friend have smoked?�� and ��Have you ever considered making attempt at smoking?�� The contents validity of the questionnaire was based on the literature and was assessed by an expert committee panel. Statistical analysis Frequency and percentage were used to describe the distribution of HSR and smoking initiation stratified by parental smoking behaviour. We used ��2 tests to examine the differences between HSR and smoking initiation. We then applied a Breslow-Day homogeneity test to assess whether the relations between HSR and smoking initiation were modified by parental smoking behaviours.