Measures
Two dependent variables from the YRBS were created. The first captured past 30-day use of e-cigarettes and was a dichotomous indicator equal to 1 for respondents who used e-cigarettes at least 1 day in the past 30 days and was equal to zero, otherwise. The second captured current e-cigarette use intensity, a quasi-continuous measure of the number of days participants used e-cigarettes during the past 30 days on the basis of the midpoints of the categorical responses to the question, During the past 30 days, on how many days did you use an electronic vapor product? as follows: 1.5 (1–2 days), 4 (3–5 days), 7.5 (6–9 days), 14.5 (10–19 days), 24.5 (20–29 days), and 30 (all 30 days).
Using other items from the survey, several independent variables believed to affect e-cigarette use among 9th–12th-grade students were constructed. These variables include sex (male and female [reference]), age, grade level (Grades 10, 11, 12, and 9 [reference]), and indicators for race and ethnicity (non-Hispanic Black, non-Hispanic American Indian or Alaskan Native, non-Hispanic Asian, non-Hispanic Native Hawaiian or Pacific Islander, non-Hispanic multiple races, Hispanic, and non-Hispanic White [reference]).
Real state-level sales-weighted average price of 1 mL of e-liquid for the first 6 months of each year was merged with YRBS data. This variable was created using NielsenIQ Retail Scanner Sales data. NielsenIQ captures universal product code data from participating independent, chain, and gas-station convenience stores; food, drug, and mass merchandizers; discount and dollar stores; and military commissaries from 23 U.S. states that are commercially available: Alabama, Arizona, California, Colorado, Florida, Georgia, Illinois, Indiana, Louisiana, Kentucky, Massachusetts, Michigan, Missouri, New Jersey, New York, North Carolina, Ohio, Oregon, Pennsylvania, Tennessee, Texas, Virginia, and Washington. These states account for 78% of the population as of 2019 and approximately 79% of all e-cigarette sales dollars tracked by NielsenIQ. Any missing data from the remaining states were dropped. The sales-weighted average price per 1 mL of e-liquid was calculated by first identifying milliliters of e-liquid for each of the unique universal product codes in the data set. The analysis focused exclusively on products that contain e-liquid and excluded hardware, batteries, and starter kits with no e-liquid (6.5% of the data). The data were supplemented through online searches on e-liquid milliliters, finding information for 94% of all barcodes. To calculate the sales-weighted average price of 1 mL of e-liquid, total sales dollars were summed at the state level and averaged over the first 6 months to coincide with the timing of the survey and then divided by total milliliters sold and adjusted for inflation to January 2020 dollars using the U.S. Bureau of Labor Statistics Consumer Price Index. Price is inclusive of all taxes levied, with the exception of sales taxes.
Standardized state-level e-cigarette tax data were merged using publicly available e-cigarette tax data from Cotti et al.,
24- Cotti C
- Nesson E
- Pesko MF
- Phillips S
- Tefft N.
Standardising the measurement of e-cigarette taxes in the USA, 2010–2020.
who used Nielsen retail scanner data and e-cigarette product characteristics to develop a method to standardize e-cigarette taxes as an equivalent average excise tax rate measured per milliliter of e-liquid fluid. This variable measures both the magnitude and structure of the various e-cigarette taxes across states. From CDC's State Tobacco Activities Tracking and Evaluation System, state-level policies on e-cigarette bans in private work sites and minimum legal purchase age laws for e-cigarettes were merged, creating dichotomous indicators of 1 for students living in U.S. states with these policies in place and zero for those living in states without such policies.
Statistical Analysis
A 2-part demand model developed by Cragg was used.
25Some statistical models for limited dependent variables with application to the demand for durable goods.
First, a 2-way fixed-effects logit regression to estimate the effect of e-cigarette taxes or e-cigarette prices, private worksite bans, and minimum legal age requirements on past 30-day use of e-cigarettes was used (
n=39,233). Second, a 2-way fixed-effects generalized linear model with log-link and gamma distribution to estimate the effects of the same covariates on e-cigarette use intensity was used (
n=9,228). This 2-way fixed-effects approach used state-fixed effects to control for time-invariant unobserved state-level factors and year-fixed effects to control for changes in the distribution of e-cigarette use by high-school students over time. All models used robust standard errors clustered on the interaction of the primary sampling unit and year.
Table 1 contains estimates from past 30-day e-cigarette use equations, and
Table 2 contains estimates from e-cigarette use intensity equations. Six models were estimated for each dependent variable. The first model included real standardized e-cigarette taxes, age, sex, grade level, race/ethnicity, year-fixed effects, and state-fixed effects. Model 2 was identical to Model 1 but included private worksite e-cigarette bans. Model 3 was identical to Model 2 but included the minimum legal purchase age of 18 years. Models 4–6 were identical to Models 1–3 but replaced the real standardized tax with the real price of 1 mL of e-liquid. Including only tax or price in Models 1 and 4 and no other e-cigarette restriction policies reduces collinearity from potentially correlated measures. When states enact policies on e-cigarettes, they often enact several initiatives simultaneously. This can also be true of states with anti–e-cigarette sentiment, where the statuses of policies are correlated to certain points in time. However, omitting e-cigarette policies in Model 1 may lead to biased estimates of the effects of taxes or prices on high school e-cigarette use.
Table 1Logistic Regression Model Predicting Past 30-Day E-Cigarette Use
Note: Boldface indicates statistical significance (*p<0.10, **p<0.05, and ***p<0.01).
All equations include an intercept and dichotomous indicators for each state in the sample minus one. t-statistics are presented in paratheses.
MLPA, minimum legal purchase age.
Table 2Generalized Linear Regression Model Predicting Intensity of E-Cigarette Use
Note: Boldface indicates statistical significance (*p<0.10, **p<0.05, and ***p<0.01).
All equations include an intercept and dichotomous indicators for each state in the sample minus one. t-statistics are presented in paratheses.
MLPA, minimum legal purchase age.
Because the analytical methods were nonlinear, the estimated parameters do not provide effect magnitude. Therefore, tax and price elasticities of demand—a measure of price sensitivity—are in
Table 3.
Table 4 uses the estimates from the 2-part demand models to simulate past 30-day e-cigarette use and intensity rates under alternative assumptions where both tax and prices were to increase by $0.50 and $1.00 per milliliter of e-liquid.
Table 3Tax and Price Elasticities of Demand
Table 4Simulations of Past 30-Day E-Cigarette Use and Intensity of Use Given $0.50 and $1.00 Increases in Tax and Price of E-Cigarette Products
Notably, traditional cigarette taxes are not included as determinants of e-cigarette demand in the models. In diagnosis models (not shown), the inclusion of traditional cigarette taxes, state-fixed effects,and year-fixed effects explained 94% of all within-state variation in e-cigarette taxes. Moreover, e-cigarette taxes, state-fixed effects, and year-fixed effects were regressed on state and federal traditional cigarette taxes (not shown). Variation in traditional cigarette taxes was fully explained (98%) by e-cigarette taxes, state-fixed effects, and year-fixed effects, indicating that there was not enough independent variation in traditional cigarette taxes after controlling for e-cigarette taxes. Finally, we regressed models (not shown) with no state-fixed effects, which yielded negative and significant effects of e-cigarette taxes on e-cigarette prevalence but insignificant effects of traditional cigarette taxes. These diagnosis models confirmed that the exclusion of traditional cigarette tax results in more robust models.