Howard et al. BMC Public Health 2011, 11:68 http://www.biomedcentral.com/1471-2458/11/68 Table 1 Descriptive statistics for school variables in the analysis (n = 879) Percentage Mean (SD) Min Max Percentage Missing Prior to Imputation Page 4 of 8 0 8.8 0 0 0 0 0 0 0 0 0 0 0 6.4 Dichotomous variables Retailersa Fast Food Convenience Supermarket School location Non-urban Urban Continuous variables Number of students per school % Overweight % Male % Female % White, non-Hispanic % Hispanic/Latino % African-American % Asian % Native American % Pacific Islander % Declined to state ethnicity % Free/reduced meals 50.4 28.6 23.8 52.6 38.7 474.2 (229.6) 30.7 (10.4) 50.9 (4.5) 49.1 (4.5) 32.8 (25.6) 45.2 (27.3) 7.3 (9.4) 12.0 (14.8) 0.84 (1.4) 0.7 (1.0) 1.5 (3.0) 42.1 (26.0) 100 3.0 26.6 19.0 0 1.6 0 0 0 0 0 0 1486 75.4 81.0 73.4 91.4 100.0 79.0 86.8 14.9 8.6 45.7 100.0 a At least one such retailer within an 800 m network buffer. students on the California Physical Fitness Test in 2007 (659 schools reporting scores for fewer ninth grade students were excluded from the analysis; 479 of these excluded schools reported overweight scores, with a mean rate of 33.8%). The number of students per school ranged from 100 to 1486. The percentage of students that were overweight in each school ranged from 3.0 to 75.4%, with a mean of 30.7%. Although this is a substan- tial range, it is not surprising given the heterogeneity in ethnic and income and composition of these schools (e.g. up to 91.4% white, non-Hispanic, or 100% Hispa- nic/Latino). Half of the study schools were located within an 800 m network buffer of a fast food restau- rant. The other two types of retailers examined were less prevalent, with 28.6% of schools located near conve- nience stores, and 23.8% near supermarkets. Correlations Rates of overweight ninth grade students were positively associated with the presence of nearby convenience stores and fast food restaurants, as shown in Table 2. Convenience stores demonstrated stronger correlations with school overweight rates (.138) than fast food res- taurants (.091). Supermarkets demonstrated no associa- tion with school overweight rates, however. Schools with higher percentages of Latino students were more likely to be near convenience stores (.137) and fast food restaurants (.101), but were not more likely to be near supermarkets (.020). Schools with more low-income students, as measured by the percentage receiving free & reduced price meals, demonstrated very similar correlations (.131 with convenience stores, .108 with fast food, .003 with supermarkets). Schools located in non-urban areas were less likely to be near fast food restaurants (-.137) and had lower rates of over- weight students (-.083) when compared to urban schools. Regression The first model regresses the percentage of overweight ninth grade students on the three retail variables, as shown in Table 3. It indicates that controlling for the influence of these retailers simultaneously, the presence of a convenience store within an 800 m network buffer of a school is predicted to increase the percentage of overweight students by 3.5% (95% confidence interval 1.9, 5.2). For a typical-sized school (474 ninth grade stu- dents), for example, approximately 17 additional stu- dents in this grade would be expected to fall in the overweight category in comparison to school without a nearby convenience store. The presence of a fast food restaurant within this same distance is predicted to increase the percentage of overweight students by 1.8% (95% confidence interval 0.2, 3.4), approximately half the strength of the convenience store association. The pre- sence of a supermarket within this buffer is predicted to