Baseline Characteristics and Generalizability of Participants in an Internet Smoking Cessation Randomized Trial.

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The potential for sampling bias in Internet smoking cessation studies is widely recognized. However, few studies have explicitly addressed the issue of sample representativeness in the context of an Internet smoking cessation treatment trial. The purpose of the present study is to examine the generalizability of participants enrolled in a randomized controlled trial of an Internet smoking cessation intervention using weighted data from the National Health Interview Survey (NHIS). A total of 5290 new users on a smoking cessation website enrolled in the trial between March 2012 and January 2015. Descriptive statistics summarized baseline characteristics of screened and enrolled participants, and multivariate analysis examined predictors of enrollment. Generalizability analyses compared demographic and smoking characteristics of trial participants to current smokers in the 2012-2014 waves of NHIS (n = 19,043) and to an NHIS subgroup based on Internet use and cessation behavior (n = 3664). Effect sizes were obtained to evaluate the magnitude of differences across variables. Predictors of study enrollment were age, gender, race, education, and motivation to quit. Compared to NHIS smokers, trial participants were more likely to be female, college educated, and daily smokers and to have made a quit attempt in the past year (all effect sizes 0.25-0.60). In comparisons with the NHIS subgroup, differences in gender and education were attenuated, while differences in daily smoking and smoking rate were amplified. Few differences emerged between Internet trial participants and nationally representative samples of smokers, and all were in expected directions. This study highlights the importance of assessing generalizability in a focused and specific manner. CLINICALTRIALS.GOV: #NCT01544153.