Method Participants The ITC Four country survey encompasses longitudinal representative cohorts of adult smokers across the United States, Canada, United method Kingdom, and Australia. For a full description of the methodology and the conceptual framework of the ITC project, see Thompson et al. (2006) and Fong et al. (2006), respectively. For the current study, respondents were eligible if they reported making a quit attempt in the interval between Waves 4 and 5 and/or Waves 5 and 6 of the ITC Four country survey and were either daily smokers or quit at the prior wave (4 or 5). As replenishment of the sample is only from smokers, respondents were not eligible at their first survey. This gave 1,880 and 1,866 respondents at Waves 5 and 6, respectively. There were 724 respondents who reported a quit attempt at both waves.

Respondents were excluded from the analyses of 6-month abstinence if they had missing data on any of the covariates or could not recall whether there was any delay between the decision to quit and implementation of their most recent quit attempt. Respondents were included in the 6-month outcome analyses if they had either started their most recent quit attempt 6 months or more before the survey or began 6 months or less before the survey and were followed up at the next wave to determine outcomes for those quit at the reporting wave. To avoid a bias toward including more recent failed attempts, we excluded those who had began their most recent quit attempt less than 6 months ago and had returned to smoking at the reporting wave but were not followed up.

As such, there were 1,462 cases available for the analyses of the outcome at Wave 5 and 1,376 for the outcome at Wave 6. This means that there are 25%�C30% fewer cases in the 0- to 6-month time since quit for the analyses by outcome compared with the analyses of prevalence of spontaneous attempts. Measures Sociodemographic variables Demographic variables included: age (18�C24, 25�C39, 40�C54, and 55+ years), sex, country, and socioeconomic status (SES). SES was derived from separate measures of income and education that were classified into within-country tertiles (low, moderate, and high). The mean of income and education was used to estimate a three-level composite SES variable. Therefore, low SES corresponds to low�Clow and low�Cmoderate combinations, and high SES corresponds to moderate�Chigh and high�Chigh combinations. Anacetrapib Moderate SES corresponds to all other combinations of income and education. Where respondents refused to give their income (n = 123 at Wave 5 and n = 117 at Wave 6), only education was used to estimate SES.