Participants were drawn from a registry of adolescents originally developed from a national phone survey conducted by the Center for Regulatory Research on Tobacco Communication in 2014–2015 [ 18 ]. In order to draw an at-risk sample for this pilot study, we mailed letters that included a2 bill to a sub-sample of adolescents (= 200) who were susceptible to or had used any tobacco product at the time of the 2014–2015 survey. Those interested in participating went to our website and were screened for participation after entering their unique participant code provided in the letter. Inclusion criteria for this pilot study were: currently aged 14–18, had a smartphone, and agreed to send and receive text messages with us. The criteria included having a smartphone because this pilot involved a text-messaging feasibility component that is reported in a separate paper [ 19 ]. Two weeks after the original letter was sent, we mailed a second letter (with no cash) to adolescents who had not yet taken the screening survey. These recruitment efforts yielded 88 prospective participants who took the screening survey. Five were ineligible (one did not have a smartphone, four did not want to send and receive multiple text messages) and 14 were eligible but chose not to enroll. Thus, our final sample was= 69.

2.2. Measures

E-cigarette knowledge . E-cigarette knowledge was assessed using six items. Participants were asked if they knew that e-cigarettes: (1) usually contain nicotine, an addictive chemical; (2) use liquid that contains harmful chemicals; (3) may harm teen brain development; (4) have unknown long-term health effects; (5) are not risk-free; and (6) use liquid that is made from tobacco. Participants answered “true,” “false,” or “don’t know.” Responses were dichotomized into correct (True—1) versus other (False or don’t know—0) answers and summed to create a composite knowledge score, with a higher score indicating greater knowledge.

Tobacco product susceptibility. E-cigarette susceptibility was assessed using five items [ E-cigarette susceptibility was assessed using five items [ 20 21 ]; participants were asked, “do you think that…” followed by example items such as “you will use an e-cigarette or other vaping device soon?” and “if one of your best friends were to offer you an e-cigarette or other vaping device, would you use it?” Cigarette susceptibility was assessed using the latter ‘best friend’ item only. Responses were on a four-point scale from “definitely no” to “definitely yes.” Participants were susceptible if they answered anything other than “definitely no” to any of the questions.

Risk beliefs—perceived risks. Perceived risks was adapted from an existing scale [ r = 0.66). Perceived risks was adapted from an existing scale [ 22 ] and began with the stem, “If I were to use an e-cigarette or other vaping device, I would…” and initially contained 10 items. An exploratory factor analysis of the items revealed that they factored into three dimensions: health worry (three items—e.g., “worry about my health”), health consequences (three items—e.g., “harm my lungs”), and addiction (two items—e.g., “get addicted”). Two items loaded complexly (i.e., loaded on several different factors) and were dropped. Responses were on a five-point scale from “definitely wouldn’t” to “definitely would.” Coefficient alpha of the full 8-item scale was α = 0.86, while for the subscales was as follows: health worry (α = 0.89), health consequences (α = 0.87), and addiction (= 0.66).

Risk belief—perceived relative risk. Perceived relative risk was measured using one item that asked participants to rate the risks of using e-cigarettes compared to smoking combustible cigarettes on a five-point scale ranging from “much less harmful” to “much more harmful”.