摘要：Objectives . We examined whether retail tobacco outlet density was related to youth cigarette smoking after control for a diverse range of neighborhood characteristics. Methods. Data were gathered from 2116 respondents (aged 11 to 23 years) residing in 178 census tracts in Chicago, Ill. Propensity score stratification methods for continuous exposures were used to adjust for potentially confounding neighborhood characteristics, thus strengthening causal inferences. Results . Retail tobacco outlets were disproportionately located in neighborhoods characterized by social and economic disadvantage. In a model that excluded neighborhood confounders, a marginally significant effect was found. Youths in areas at the highest 75th percentile in retail tobacco outlet density were 13% more likely (odds ratio [OR]=1.13; 95% confidence interval [CI]=0.99, 1.28) to have smoked in the past month compared with those living at the lowest 25th percentile. However, the relation became stronger and significant (OR=0.21; 95% CI=1.04, 1.41) after introduction of tract-level confounders and was statistically significant in the propensity score–adjusted model (OR = 1.20; 95% CI = 1.001, 1.44). Results did not differ significantly between minors and those legally permitted to smoke. Conclusions . Reductions in retail tobacco outlet density may reduce rates of youth smoking. Although recent evidence has shown that cigarette use has been declining over the past decade, 1 , 2 the proportion of smokers remains high. From a public policy standpoint, retail access to cigarettes is often assumed to be an important determinant of smoking behavior. Reducing the number of retail outlets is a plausible strategy to reduce smoking among all age groups. 3 To curb underage smoking, limiting the number of outlets within an area might be used in tandem with better law enforcement of youth purchasing bans. The latter, although potentially important, is not completely effective, because when legal age limits are strictly enforced, minors may seek out adult smokers to procure cigarettes in local stores. 2 In addition to providing more opportunities to purchase cigarettes, areas with a higher density of retail tobacco outlets may also encourage use by increasing levels of exposure to point-of-sale advertising in the form of prominently placed posters and banners that display information on available brands and sale prices. Because recent legislation limits event promotions and media advertisements, the retail arena is one of the few remaining channels that cigarette manufacturers can use to target both minors and those legally permitted to purchase cigarettes. 4 Evidence on the efficacy of merchant education and better law enforcement as a means of reducing youth smoking is not encouraging. (We use the term youth to span the developmental periods from childhood through early adulthood.) Much of what is known comes from randomized community trials, which restrict retail access in intervention communities through the use of merchant education programs or compliance checks using underage confidants to enforce purchasing bans for minors. 5 – 7 Summarizing findings from these randomized community experiments, Fichtenberg and Glantz 3 conducted a meta-analysis of 9 studies published between 1985 and 2001 and found no evidence favoring the efficacy of such approaches. Although DiFranza 8 criticized the lack of comparability of interventions in the studies summarized, these results cast some doubt on the viability of using enhanced legal compliance alone to reduce youth smoking. 3 , 8 , 9 If reducing legal access has had limited effects, one might reason that reducing the density of outlets could be a more effective strategy for curtailing youth smoking. The evidence here is also extremely limited. In an observational study, Pokorny et al. 10 reported no association between retail tobacco outlet density and adolescent cigarette use. However, their study had access to data from only 11 local communities, limiting statistical power and generalizability. Thus, additional research is needed to identify the effect of local outlet density on youth smoking. A challenge to such a research agenda is that, unlike studies that increase merchant education and legal compliance, studies in which communities are randomly assigned to receive different levels of retail outlet density are difficult to construct. As a result, researchers must devise effective strategies for observational studies that approximate the design features of a randomized experiment. These include measuring and controlling a host of community characteristics that would otherwise be confounded with retail outlet density. Two techniques are commonly used to reduce confounding in nonexperimental studies of causal effects: statistical control (e.g., with multiple regression) and case-control matching (i.e., selecting case and control communities that are thought to be comparable in terms of potential confounding variables). Statistical control may lead to overparameterization if the number of potential confounders is large relative to the number of study units, as is likely to be the case in community-level studies. On the other hand, case-control matching is often difficult, as it may require a large number of units to find exact matches on even a modest number of covariates. A useful alternative strategy that does not suffer from these limitations is propensity score stratification. 11 The idea is to create subsets of units (i.e., neighborhoods) that are very similar on the likelihood or “propensity” to receive a treatment (i.e., exposure to retail tobacco outlets) given a set of background covariates. Ideally, each subset contains a mixture of units that did and did not actually receive the treatment. Treatment effects are then estimated within each subset and averaged to obtain an overall average effect. Adjusting for the propensity score in this way provides an effective means to control for a large number of confounders without overfitting the model that predicts the outcome. 12 We used both standard regression and propensity scoring stratification with a sample of youths living within geographically diverse neighborhood communities in Chicago. By comparing models that use different methods of statistical control to reduce the effect of community-level confounders, we were able to examine the stability of the causal influence of retail tobacco outlet density on youth smoking. In addition, we tested whether the effect of exposure varied between minors and those legally permitted to buy cigarettes to determine whether tobacco control strategies that target retail outlet density would have a differential impact between minors and those of legal age.
