“…Residual confounding might occur due to not obtaining social factors, such as social class, single-parent status, smoking behavior of parents, siblings and peer groups, or second-hand smoking (Bailey et al, 2006;Bidstrup et al, 2009;Biglan et al, 1995;Fergusson, Lynskey, & Horwood, 1995;Go et al, 2010;Keyes et al, 2008;Mercken et al, 2010;Milton et al, 2008;O'Loughlin et al, 2009;Otten et al, 2007), Additional internal factors like poor academic performance, school absenteeism, violent or criminal behavior, illicit drug use, risky sexual behavior, or susceptibility to tobacco advertising might be other potential factors (Conwell et al, 2003;Gohlmann et al, 2010;Mason & Windle, 2001;O'Loughlin et al, 2009). Unfortunately, these variables were not obtained as potential confounders in this study.…”