Howard et al. BMC Public Health 2011, 11:68 http://www.biomedcentral.com/1471-2458/11/68 Discussion This study indicated that schools within 800 m of con- venience stores had higher rates of overweight students than schools located farther from these retailers. This association was independent of ethnic, socioeconomic and gender composition, as well as urban/non-urban location. It extends similar findings reported in two stu- dies of children that focused on a different context, retailers surrounding homes. One involved 8th and 10th grade students nationally, [11] and the other focused on 6-8 year old children in East Harlem, NY [10], but both reported that residing near convenience stores was asso- ciated with a higher body mass index. Our analysis also underscores the importance of socio- economic variables on the risks of being overweight. We observed that 1) the percentage of Hispanic/Latino stu- dents, 2) the percentage of students receiving free & reduced price meals, and 3) the percentage of African- American students, demonstrated much stronger asso- ciations with school overweight rates than the other variables that we tested. Importantly, the first two of these variables were associated with an increased likeli- hood that a school would be located near a convenience store. While inequalities in retail food environments outside of schools were not a focus of this study, the results provide further support for income and ethnic disparities reported by others [31,34,35,38]. The poten- tial risks of having convenience stores near schools, even if substantially smaller than known individual/ household-level risks by comparison, are nonetheless disproportionately borne by those already at much higher risk of being overweight. While two recent studies found that residing near supermarkets was associated with a lower risk of over- weight, [11,12] our study of the environments sur- rounding schools did not find an association between the proximity of these retailers and rates of student overweight. Also, in contrast to two previous studies in California [24,25], we failed to find a consistent asso- ciation between school overweight rates and nearby fast food restaurants. Our results therefore provide only partial support for theories that the availability of low-energy, nutrient-dense foods and/or low-nutrient, energy-dense foods contributes to differing consump- tion patterns and weight outcomes in students. Some of the limitations of our approach, described below, may account for why these expected associations were not observed, however. Study limitations Our study has a number of limitations. Its ecological design limits inferences about school food environments’ potential relationships with individual-level behaviors/ Page 6 of 8 outcomes. Even at the school-level we lacked data for variables known to influence food/beverage consumption and physical activity, such as the type of food service [39], presence or absence of vending machines [40], open/closed campus [31], mode of transportation (e.g. buses), and physical education requirements [41]. In addition, the cross-sectional design did not allow for ana- lysis of the time order of exposures and the outcome. Another potential limitation was the exclusion of schools with small numbers of students due to unstable rates. Our sources of data are prone to error, such as those we utilized to classify and geographically locate the retailers, as well as the methods and standards used to determine student overweight status. This error may bias the esti- mates toward or away from the hypothesis. Study strengths Our study also has several strengths. We utilized net- work buffers to more accurately model retailer distance to schools when compared to studies that have employed Euclidean buffers. We simultaneously exam- ined associations between three different retail types and the outcome of overweight, rather than focusing on a single type of retailer. Finally, our outcome measure of student overweight, although aggregated at the school- level, was based on criterion-referenced, test-specific measurements, instead of less accurate student self- reported height and weight. Suggestions for future research Future studies could address some of the limitations of our analysis by ground-truthing retail location data, as well as incorporating additional school-level variables that may influence student energy intake and expendi- ture. Subsequent efforts could also benefit from charac- terizing the quantity and types of foods available within restaurants and stores, rather than relying on retailer categories, as otherwise similar establishments may differ greatly in their availability of healthy and less healthy foods [42,43]. Additional methodological improvements could include longitudinal designs and/or cross-classified multilevel models [44]. The latter could incorporate the retail food environments of both schools and residences at the macro-level, and examine how these interact with individual student health outcomes and behaviors, as in a recent study conducted by Laska et al. with adolescents in Minneapolis/St. Paul [26]. Conclusions This ecological study in California found that nearby convenience stores were associated with higher school rates of overweight students, after controlling for socioe- conomic and ethnic factors known to be linked to this