Dickerson et al. BMC Public Health 2011, 11:926 http://www.biomedcentral.com/1471-2458/11/926 Table 2 Simultaneous Quantile Regression Results–by Body Mass Index Classification (Continued) Obese Class III (n = 236) Age -0.25 Sex (1 = Male, 2 = Female) 2.92 Non-Hispanic White 4.64 African-American 12.14 Hispanic 1.98 Rurality (0 = Urban, 1 = Rural) 1.36 Educational Attainment (1 = First Grade, 17 = Graduate School) -0.62 Minutes per week of moderate physical activity -0.0020 Hours per day spent watching television 1.87 Constant 50.91 0.06 1.17 2.64 4.74 2.39 1.14 0.26 0.0006 0.45 6.48 -4.12 2.50 1.76 2.56 0.83 1.19 -2.35 -3.76 4.18 7.85 <0.001 -0.37 0.012 0.63 0.079 -0.53 0.011 2.85 0.407 -2.70 0.233 -0.88 0.019 -1.14 <0.001 -0.0040 <0.001 0.99 <0.001 38.20 Page 8 of 10 -0.13 5.21 9.81 21.44 6.66 3.60 -0.10 -0.0010 2.74 63.62 the differences between BMI classifications were statisti- cally significant. Figure 2 illustrates the effect of age, sex, education, and rurality on BMI classification. All classifications of age were statistically significant. The difference between overweight and obese class I demonstrated negative het- erogeneous effect. The difference between obese class I and obese class II also demonstrated negative heteroge- neous effect. The effect of sex on BMI was statistically significant for normal weight, overweight, and obese class III. The difference between obese class II and obese class III demonstrated positive heterogeneous effect. The effect of educational attainment on BMI was statistically significant for all BMI classifications except normal weight and overweight. The difference between normal weight and overweight demonstrated negative heterogeneous effect. The effect of rurality on BMI was only statistically significant for the overweight classification. Discussion The effects of PA and sedentary behavior on BMI classification are not homogeneous This study has addressed the concept of sedentary beha- vior and PA by looking specifically at how watching tel- evision and engaging in moderate levels of PA effect BMI classification. The effect of watching television more than doubled between the normal weight classifi- cation and the overweight classification. For those in the overweight classification, an extra 2 h spent watching television per day was associated with nearly one addi- tional BMI point. But, this finding was not consistent across all BMI classifications. Watching television was only incrementally detrimental to BMI when viewed from the perspective of moving from normal weight to overweight. Among respondents who were already over- weight, a similar effect was not observed. Conversely, the effect of moderate PA on BMI was greater for those in higher BMI classifications than for those in lower BMI classifications. Specifically, the transition from obese class I to obese class II demonstrated a significant decrease in BMI with increased moderate PA (F = 4.54, p = 0.033). This finding suggests there is much to be gained from engaging individuals with higher BMI in moderate PA. Our findings about the heterogeneous effect of PA is somewhat supportive of Cooper and colleagues’ [28] finding that adults of differing BMI classifications also had differing levels of PA. Our finding supplements their study by isolating the statistical influence of differ- ing levels of PA on BMI at different BMI classifications. Socio-demographic effects on BMI classification are noteworthy This study demonstrates the significant association of socio-demographics with BMI classification. Self-report- ing as African-American had a significant effect on BMI classification which increased as BMI increased. For those in the overweight BMI classification, self-reporting as African-American was associated with nearly four additional BMI points. This effect was nearly four times as great as the effect of self-reporting as African-Ameri- can in the normal weight BMI classification. Conversely, self-reporting as Hispanic was only significantly asso- ciated with lower BMI classifications such as normal weight, overweight, and obese class I. However, the effect of self-reporting as Hispanic was homogenous across these categories. Age was significantly associated with all BMI classifi- cations. Among higher levels of BMI classification, the effect of increasing age became stronger. For those in the obese class III BMI classification, each additional year aged was associated with a reduction of one quarter of a BMI point. Self-reporting as male was also signifi- cantly associated with lower BMI among the two lowest BMI classifications, normal weight and overweight. However, among the highest BMI classification, obese class III, self-reporting as female was significantly asso- ciated with higher BMI. The difference in the effect of being female on BMI was also significantly higher for