Study
The Farrelly and colleagues 2005 study used a pre?post quasi-experimental design that relates changes in youth smoking prevalence to varied exposure to the truth? campaign over time and across media markets in the United States. The study used media delivery data as well as data from the Monitoring the Future (MTF) surveys during 1997?2002. The MTF survey is given annually in the spring to a nationally representative sample of roughly 50,000 8th, 10th, and 12th grade students and asks questions designed to monitor alcohol, tobacco, and illicit drug use among youths.
The study outcome was a dichotomous variable for reporting any quantity of smoking in the past 30 days, based on the following question from the MTF survey: how frequently have you smoked cigarettes in the past 30 days? The variable was coded as zero for students who answered ?none per day,? and was coded as 1 for students who answered ?less than 1 cigarette,? ?1 to 5 cigarettes,? ?about ? pack,? ?about 1 pack,? ?about 1? packs,? or ?2 packs or more per day.? The years 1997?99 represented a precampaign study period. The years 2000?02 represented the years the campaign was launched nationally, although the dose of campaign messages varied considerably across media markets and over time.
Student?s exposure to the truth? campaign was measured by cumulative gross rating points (GRPs) in each of the 210 television markets in the United States. GRPs measure the total volume of delivery of a media campaign to a target audience. They are equal to the percentage of the target audience that is reached by the campaign times the frequency of exposure. Student?s exposure was defined as the cumulative number of truth? campaign GRPs that were delivered in a school?s media market from the beginning of the campaign in February 2000 to the time of the MTF survey in the springs of 2000, 2001, and 2002. Students from 1997?99 served as a historical unexposed comparison group (GRP=0).
Potential confounding variables were controlled for, including individual-level variables (grade, race/ethnicity, gender, parental education), media market-level variables (median household income, percentage of the population who were college graduates, population size), and state-level variables (inflation-adjusted cigarette prices, investments in tobacco control programs). The study used population average logistic regression models to estimate current youth smoking prevalence as a function of individual-, media market?, and state-level influences. Regressions were estimated by combining the cross-sectional MTF surveys from 1997 through 2002 to relate the odds that an individual smoked to the media market dose of the campaign, measured at the time of the survey.
Study
A second study by Farrelly and colleagues (2009) used a similar quasi-experimental design to relate changes in smoking initiation to levels of exposure to the truth? campaign over time and across media markets in the United States using longitudinal data. The study analyzed data from the National Longitudinal Survey of Youth 1997 (NLSY97). The NLSY97 is designed to provide information about youths? transition from school to the labor market and into adulthood. Members of the longitudinal cohort were interviewed annually beginning in 1997 and were ages 15?20 when the truth? campaign began in 2000. Data from Rounds 1?8 (1997?2004) were used in the analyses. The baseline sample included a national representative sample of 8,904 adolescents between the ages of 12 and 17 years during the initial survey round in January 1997.
The outcome measure included the age of initiation of trying smoking based on three questions from the NLSY97. All youths were asked: have you ever smoked a cigarette? If youths answered ?yes,? they were asked a follow-up question to determine the age of initiation: how old were you when you smoked your first cigarette? For subsequent surveys, youths were asked: have you smoked a cigarette since the last interview? If youths answered ?yes? to this question, the age of initiation was based on their age at the time of the survey.
The authors again used gross rating points (GRPs) to quantify the reach and frequency of exposure to the campaign on television. The cumulative sum of exposure (the GRPs) was calculated for each study participant for each wave of the survey, based on the participant?s media market of residence each year. For example, if a market received 1,000 GRPs per year, cumulative exposure would be 5,000 GRPs by 2004 for a cohort member who lived in this market for all years since the campaign launch in 2000. The precampaign period before February 2000 (1997?99) was set to zero (GRP=0).
Several confounding variables were controlled for in the study, including individual/family confounders (race/ethnicity, gender, total individual income, living with both parents, ever being suspended from school, etc.), market-level characteristics (average disposable family income, average high school completion rates, and percentage of population living in rural areas), and time-varying confounders (such as cumulative GRPs, annual inflation-adjusted per capita state-level funding for tobacco control programs, and state cigarette prices). The study used discrete-time survival analysis to assess the influence of the truth campaign on smoking initiation. The process allowed for the calculation of the probability that an individual will initiate smoking for each age represented in the sample, given that he or she had not previously begun smoking.