So BMC Public Health 2012, 12:258 http://www.biomedcentral.com/1471-2458/12/258 Table 1 The characteristics of the subjects Variables Age (years) Height (cm) Weight (kg) Body mass index (kg/m2) Family income status Father’s education level Mother’s education level Academic performance Grade Very rich Rich Average Poor Very poor Unknown* Middle school or lower High school College or higher Unknown* Middle school or lower High school College or higher Very good Good Average Bad Very bad 7th 8th 9th 10th 11th 12th Data are expressed as mean ± SD or N (%). * These records were omitted from the multivariate logistic regression analysis in Table 2. responses, the participants were divided into 2 groups: (1) those who reported below-average academic perfor- mance and (2) those who reported average or above- average academic performance. Covariate variables Age: The age of the students defined by the KYRBWS-V data was used without any modification. Body mass index (BMI): The students were asked to report their height and weight, and their BMI (kg/m2) was calculated from the data supplied by each student. Father’s education level: The 4 response options were (1) unknown, (2) middle school or lower, (3) high school, and (4) college or higher. Data from students reporting “Unknown” were omitted from the multivariate logistic regression analysis. Mother’s education level: The 4 response options were (1) unknown, (2) middle school or lower, (3) high school, and (4) college or higher. Data from students reporting “Unknown” were omitted from the multivariate logistic regression analysis. Family income status: The 5 response options were (1) very rich, (2) rich, (3) average, (4) poor, and (5) very poor. Data analysis All results from this study are shown as mean ± standard deviation (SD) values. The statistical software SPSS Com- plex SampleTM version 18.0 (SPSS, Chicago, IL, USA) was used in all the analyses to evaluate the complex sample survey. For statistical assessments, we took into consider- ation the complex sampling design of the KYRBWS-V. To Boys(n = 39,612) 15.00 ± 1.73 Girls(n = 35,454) 15.12 ± 1.77 Total(n = 75,066) 15.06 ± 1.75 169.58 ± 8.19 60.14 ± 11.72 20.80 ± 3.21 2,923 (7.4) 8,985 (22.7) 17,743 (44.7) 7,150 (18.1) 2,811 (7.1) 7,297 (18.4) 2,707 (6.8) 14,294 (36.1) 15,314 (38.7) 8,069 (20.4) 2,447 (6.2) 17,745 (44.7) 11,351 (28.7) 04,958 (12.5) 09,172 (23.2) 10,544 (26.6) 09,661 (24.4) 05,277 (13.3) 6,933 (17.5) 6,965 (17.6) 7,035 (17.7) 6,890 (17.4) 6,104 (15.4) 5,685 (14.4) 160.08 ± 5.39 51.47 ± 7.67 20.05 ± 2.58 1,434 (4.0) 6,908 (19.6) 17,706 (49.9) 7,090 (20.0) 2,316 (6.5) 5,321 (15.0) 2,483 (7.0) 13,915 (39.2) 13,735 (38.8) 5,059 (14.3) 2,490 (7.0) 18,212 (51.4) 9,693 (27.3) 03,454 (09.7) 08,411 (23.7) 09,675 (27.3) 09,477 (26.7) 04,437 (12.5) 5,781 (16.3) 5,903 (16.6) 5,792 (16.3) 5,587 (15.8) 6,323 (17.9) 6,068 (17.1) Page 3 of 7 165.09 ± 8.46 56.04 ± 10.91 20.45 ± 2.95 4,357 (5.8) 15,893 (21.2) 35,449 (47.2) 14,240 (19.0) 5,127 (6.8) 12,618 (16.8) 5,190 (6.9) 28,209 (37.6) 29,049 (38.7) 13,128 (17.5) 4,937 (6.6) 35,957 (47.9) 21,044 (28.0) 08412 (11.2) 17583 (23.4) 20219 (26.9) 19138 (25.5) 09714 (12.9) 12,714 (16.9) 12,868 (17.1) 12,827 (17.1) 12,477 (16.6) 12,427 (16.6) 11,753 (15.7)