Vaping duration, derived on the basis of the difference between age when respondents first used an e-cigarette and their current age, was classified as 1 of 4 mutually exclusive groups: <1 y, 1 y, 2–3 y, and >3 y.

Other tobacco products include cigarettes, cigars (cigars, little cigars, and cigarillos), smokeless tobacco (chewing tobacco, snuff, dip, snus, and dissolvable tobacco), hookahs, pipe tobacco, bidis, and heated tobacco product. Dual and poly users included respondents who reported current e-cigarette use and ≥1 d of other tobacco use in the past 30 d.

Flavor in current e-cigarette use was measured by affirmatively responding to the question “Were any of the e-cigarettes that you used in the past 30 days flavored to taste like menthol, mint, clove or spice, alcohol (wine, cognac), candy, fruit, chocolate, or any other flavor?”

Nicotine craving within 30 min of waking was measured by the question, “How soon after you wake up do you want to use a tobacco product?” with the following response options: “I do not want to use tobacco products,” “Within 5 minutes,” “From 6 to 30 minutes,” “From more than 30 minutes to 1 hour,” “After more than 1 hour but less than 24 hours,” and “I rarely want to use tobacco products.”

Four dichotomous variables were created to measure the channels of exposure to e-cigarette advertising: the Internet, newspapers and magazines, stores, and television, streaming services, or movies. Response options including “I don’t use,” “Never,” and “Rarely,” were coded as 0 (no exposure), whereas response options including “Sometimes,” “Most of the time,” and “Always” were coded as 1 (exposure). The number of exposures from multiple channels was further summed and classified into 0, 1, and 2+ types of channels.

Tobacco use by other household members was defined by the question “Does anyone who lives with you now…? (choose all that apply)” with the following response options: “Smoke cigarettes,” “Smoke cigars, cigarillos, or little cigars,” “Use chewing tobacco, snuff, or dip,” “Use electronic cigarettes or e-cigarettes,” “Smoke tobacco from a hookah or waterpipe,” “Smoke pipes filled with tobacco (not waterpipes),” “Use snus,” “Use dissolvable tobacco products,” “Smoke bidis (small brown cigarettes wrapped in a leaf),” “Use heated tobacco products,” and, “No one who lives with me now uses any form of tobacco.”

Reasons for e-cigarette use were assessed by the question “What are the reasons that you have used e-cigarettes?”

Assessed by the question “Because of a physical, mental, or emotional condition, do you have serious difficulty concentrating, remembering, or making decisions?”