Procedures for assessing and validating high and low quit interest at screening are described elsewhere (Perkins et al., 2008, 2010). Procedure As described new product below, expired-air CO and the number of cigarettes smoked were assessed daily (Mon�CFri) during the two 5-day quit attempt periods (i.e., a total of 10 daily assessments), separated by at least 1 week of ad lib smoking without any medication. These medication conditions are not particularly relevant in these analyses since our focus was on the sensitivity and specificity of CO level for verifying 24-hr abstinence, regardless of the method by which abstinence was obtained. Sessions were scheduled between noon and 4:00 p.m. at the same time of the day for each participant.

Daily expired-air CO was assessed at each session via Breathco CO monitor (Lenexa, KS) following a 20-s breath hold prior to slowly expiring air into the monitor mouthpiece (Javors et al., 2005; Raiff, Faix, Turturici, & Dallery, 2010). Subjects were not informed of their CO readings (in ppm), which were recorded away from the participants�� view. All 261 participants were randomly assigned to monetary reinforcement of daily abstinence ($12/day; n = 127) or to no reinforcement (n = 134) during both quit periods. Reinforcement was provided for those with CO <5 (��4 ppm) and who reported no smoking in the last 24hr, although none was informed of their CO values or this specific CO criterion for reinforcement. Rather, they were told they would be reinforced for ��being quit.

�� For this analysis, this manipulation enabled us to assess whether the presence or absence of reinforcement for quitting influenced the association of CO with abstinence (see Javors et al., 2005). These studies were approved by the University of Pittsburgh Institutional Review Board, and all participants provided written informed consent for participation after the nature and the consequences of the relevant study were explained. Other details of the study procedures are described in Perkins et al. (2008, 2010). Assessing Daily Abstinence or Smoking Abstinence was defined as zero cigarettes smoked in the 24hr prior to each weekday session (Monday�CFriday) during the quit attempt periods. To assess the amount of daily smoking, participants were given ��tally�� cards to monitor all cigarette use.

Tallies were returned at the next session and the number of cigarettes smoked over precisely the 24hr prior to the session was recorded, based on tallies for that day and the previous day. The rows of each tally card listed the 24 individual hours of the day (e.g., ��noon to 1:00 p.m.��), Drug_discovery with a space to mark the tallies every time they smoked a cigarette at that time. (The size and the shape of the card were intentionally designed to fit inside the cellophane wrapper of a cigarette pack, to facilitate compliance.