T.B.B. has served as an investigator in the past 5 years on research studies at the University of Wisconsin�CMadison that were funded in part by GlaxoSmithKline. T.B., B.M., and S.M.Z. are employees at Alere Wellbeing and also own stock in Alere Wellbeing (formerly Free & Rapamycin Clear, Inc.), an organization providing quitline services in Wisconsin. T.A.M. was employed by and owned stock in Free & Clear prior to being appointed Director of the Office on Smoking and Health, CDC, in September 2010. He was also an unpaid member of the Board of Directors of the nonprofit North American Quitline Consortium. T.A.M. has no current financial disclosures. M.C.F. has served in the past 5 years as an investigator on research studies at the University of Wisconsin-Madison that were funded wholly or in part by Pfizer, GlaxoSmithKline, and Nabi.

From 1997 to 2010, M.C.F. held a University of Wisconsin named Chair for the Study of Tobacco Dependence, made possible by a gift to the university from GlaxoWellcome. P.A.K., K.H.K., and D.L.F. have no financial disclosures. Acknowledgments We thank the following staff at the Center for Tobacco Research and Intervention at the University of Wisconsin School of Medicine and Public Health for their help with this research: Paul Kohn, Sandy Keller, Jared Holzhuter, Samantha Neymark, and Emily Gelber. In addition, we thank the staff at the Wisconsin Tobacco Quit Line for assistance with the study.
Complete abstinence from smoking over at least 24hr is typically the minimum duration necessary to define an ex-smoker as ��quit�� in clinical research (e.

g., Ockene et al., 2000). This duration of abstinence is supported by observations that a recent 24-hr quit predicts greater subsequent quit success (Balmford, Borland, & Burney, 2010), and any smoking at all within 24hr of trying to quit increases the risk of relapse to regular smoking (Juliano, Donny, Houtsmuller, & Stitzer, 2006; Westman, Behm, Simel, & Rose, 1997). Among those trying to quit, the majority who fail to abstain for at least 24hr will not even report the effort as a true quit attempt (Berg et al., 2010). Cotinine��s 16-hr half-life may allow verification of abstinence over the past several days (SRNT, 2002). However, a very common and immediate method of biochemically verifying 24-hr abstinence in clinical research is an expired-air carbon monoxide reading equal to or less than 8 ppm (CO < 8 ppm), or perhaps 10 ppm (Bailey, Bryson, & Killen, 2011; SRNT, 2002).

CO��s typical 4-hr half-life makes it more sensitive for verifying 24-hr abstinence. However, recent research suggests that smokers may be able to smoke one or more cigarettes within the prior 24hr and still meet such a CO criterion (��cutoff��) Batimastat for abstinence. For example, Javors, Hatch, and Lamb (2005) found that, compared with the typical criterion of ��8 ppm, a CO cutoff for abstinence of <3 ppm increased sensitivity for accurately identifying smoking (i.e.