and cohorts examined which factors predicted nicotine withdrawal class assignment in smokers. Analyses were conducted in Stata (Stata Corp, 2003), using the Huber-White Robust Variance Estimator to correct for the non-independence of measures in twins. Variables considered included: difficulty quitting, smoking persistence, seeking cessation treatment, impairment due to nicotine withdrawal, heavy smoking, HSI, major depression, social anxiety, conduct disorder, alcohol problems (dependence in the adults), and ADHD (in the adolescents only).
