Data source
The core CPS is a labor force survey conducted monthly by the US Census Bureau from a nationally representative civilian, non-institutionalized adult sample, in which households are interviewed for 4 consecutive months, rested for 8 months, and then re-interviewed for another 4 months before exiting the sample. For the TUS portion of the survey, an interview is attempted with all eligible adults in the household. If unsuccessful in obtaining the self-interview, a proxy interview is sought. The TUS-CPS is funded by the U.S. National Cancer Institute and has been conducted since 1992 approximately every 3–4 years, to provide a nationally representative cohort of smokers and nonsmokers. The rotating panel design of the CPS provided an overlapped longitudinal sample of respondents to the May 2010 TUS survey (baseline) who were re-interviewed in May 2011 (follow-up; n = 28,153). This longitudinal subsample of the TUS-CPS uses self-report respondents only. Sample survey weights are provided to account for the complex sampling design, under coverage, and non-response in the overlap sample, allowing nationally representative estimates for the U.S. [21]. Data from the TUS-CPS contain no personal identifiers and are analyzed anonymously. The Institutional Review Board at the University of California, San Diego reviewed the study protocol and did not consider it to be human subjects research.
Cigarette smoking measures
The TUS-CPS uses the standard national tobacco questions including “have you smoked at least 100 cigarettes in your entire life?” to identify ever smokers, and “do you now smoke cigarettes every day, some days, or not at all?” to identify current smokers. Average number of cigarettes smoked per day (CPD) was assessed. Change in consumption was computed as follow-up CPD minus baseline CPD, for continuing smokers. We classified those who smoked at least 15 CPD as heavier smokers, as in previous work [22].
Cessation behaviors at follow-up
A quit attempt was identified with the question “during the past 12 months, have you stopped smoking for one day or longer because you were trying to quit smoking?” or “during the past 12 months, have you made a serious attempt to stop smoking because you were TRYING to quit – even if you stopped for less than a day?”. Those who smoked fewer than 12 days in the past 30 days were alternatively asked “during the past 12 months have you tried to quit smoking completely?” Respondents who reported smoking at baseline but said they now smoked not at all at follow-up were asked “about how long has it been since you completely quit smoking cigarettes”. We use 30+ days cessation at the follow-up survey as an early marker of successful cessation [23–25]. As a sensitivity analysis, we used 30+ days cessation on the longest quit attempt of the past 12 months [26, 27], which was assessed by “during the past 12 months, what is the longest length of time you stopped smoking because you were trying to quit”. These quit attempts lasting at least 30 days were analyzed in the sensitivity analysis.
Ever use of e-cigarettes
At follow-up, ever-users of e-cigarettes were identified with the question “have you ever tried a product called an electronic or e-cigarette, such as ‘Smoking Everywhere’, ‘NJOY’, or other brands?” Use for quitting was assessed by the question “have you ever used e-cigarettes to help you quit smoking cigarettes or quit using other tobacco products?”
Use of pharmaceutical assistance at most recent quit attempt
At follow-up, those who had made a quit attempt were asked about use of the following products on the most recent attempt: “a nicotine patch, a nicotine gum or nicotine lozenge, a nicotine nasal spray or nicotine inhaler?; a prescription pill, called Chantix or Varenicline?; a prescription pill, called Zyban, Bupropion, or Wellbutrin? another prescription pill?”.
Other measures
We classified respondents into age groups (18–24 years, 25–34 years, 35–49 years, 50 years and older), as male or female, as non-Hispanic White or other race/ethnicity, and into four levels of educational attainment (less than high school, high school, some college, college and above). Using the question, “how soon after you wake up do you typically smoke your first cigarette of the day?” those who smoked within 30 min of waking were considered more dependent. Age of initiation <16 years classified a respondent as an early initiator, assessed with the question “how old were you when you first started smoking cigarettes fairly regularly”.
Statistical methods
We described the demographic and smoking characteristics among those who made any quit attempts, ever used e-cigarettes, and ever used e-cigarettes for quitting. Chi-squared tests were conducted to compare differences by e-cigarette use status. We also compared smoking characteristics by e-cigarette and pharmaceutical aid use status. We used multivariate logistic regression to examine the relationship of 30+ day cessation at follow-up with ever use of e-cigarettes, and with use of pharmaceutical aids at last quit attempt, adjusted for baseline socio-demographic characteristics and nicotine dependence levels. E-cigarette use and use of pharmaceutical aids were coded as main effects in an additive model; these use categories are not mutually exclusive. Statistical significance was assessed at the two-sided 5 % level. All statistics were computed using TUS-CPS overlap sampling weights according to recommended procedures [21], using SAS 9.3 (SAS Institute) survey procedures.