Wong et al. BMC Public Health 2011, 11:618 http://www.biomedcentral.com/1471-2458/11/618 back home’. Responses were reclassified as never/rarely/ sometimes and often/always. Environmental Travel time to school and from school were measured separately and categorised as 0-15 minutes, 16-30 min- utes and 31 minutes or more. Longer travel time implies longer distance, greater cost (in terms of temporal value), and more effort required for each mode. The same travel time implies greater (physical) effort required for active compared to passive modes (e.g., more physical effort required for walking for an hour than driving/being driven for an hour) and longer travel distance for passive than active modes. Given that par- ental decision on their child’s school travel mode mainly is based on convenience [29], travel time may be an important correlate of school travel. The likelihood of an active school travel mode is hypothesized to decrease with travel time whereas the likelihood of a passive mode is hypothesized to increase with travel time. Moving home or changing school in the past five years was modeled categorically; residential mobility may act as a latent indicator of neighbourhood familiar- ity (i.e., familiarity produces active travel). Other envir- onmental characteristics included living in an urban or rural area (based on postal codes) and school affiliation (catholic or no religion) which has been suggested as a correlate of AST [15]. Differences in religious affiliation may be a broader indicator of a difference in school transport policy between different school boards. Regio- nal variation in AST was also examined. Data analysis All analyses used Taylor series methods to account for the complex survey design. Logistic regression was used to identify the correlates of active school transport sepa- rately for the morning and afternoon and for students who mode shifted from passive in the morning to active in the afternoon. Two models were created: i) unad- justed models where the effects of each variable on active school transport or mode shift were examined separately and ii) fully adjusted models where only sig- nificant correlates in the unadjusted models were included. Physical activity and weight status as well as demographic (grade and sex) variables were included in all models. All analyses were stratified by elementary and secondary school students due to potential differ- ences in independence, availability and use of travel modes, and correlates [17]. All analyses were conducted using Stata 11. Results Tables 1 and 2 summarize the descriptive information for the sample and the prevalence of active school Page 4 of 12 Table 1 Descriptive statistics for elementary and secondary school students Elementary Secondary p-value n (%) n (%) Socio-demographic Grade Grade 7 506 (48.2) - Grade 8 583 (51.8) Grade 9 590 (21.6) Grade 10 654 (23.4) Grade 11 597 (24.3) Grade 12 638 (30.8) Sex (female) 604 (48.2) 1272 (47.9) 0.91 Weight status (overweight/obese) 241 (26.7) 620 (24.8) 0.42 Years of highest parent education 14.6 (1.6) 14.5 (1.7) 0.17 (yrs) (mean(SD)) Parent born abroad (yes) 229 (32.2) 719 (33.0) 0.83 Having sibling (yes) 1011 (92.8) 2328 (93.9) 0.44 Having car at home (yes) 1053 (95.6) 2424 (97.0) 0.13 Having driver license (yes) - 1046 (42.2) - Behavioural MVPA (≥5 days/wk) 568 (53.9) 1105 (42.6) < 0.001 Screen time (> 2 hours/day) 539 (49.6) 1518 (61.2) < 0.001 Smoking in the past 12 months 49 (5.3) 572 (22.4) < 0.001 (occasional/regular smokers) Drinking in the past 12 months 567 (52.1) 1962 (78.8) < 0.001 (occasional/regular drinkers) Part-time job Did not work outside home - 1302 (53.5) - 1-5 hours/wk - 301 (11.2) ≥6 hours/wk - 876 (35.3) Extracurricular participation (yes) 838 (77.6) 1755 (70.8) 0.06 Psychological Perceived physical health 74 (7.6) 400 (17.7) < 0.001 (excellent/good) Perceived weight status About right 779 (73.9) 1699 (65.9) < 0.001 Too thin 84 (6.3) 218 (10.2) Too fat 216 (19.8) 555 (23.9) Decision-making on mode choice 340 (3.1.5) 956 (40.4) 0.001 (often/always) Parents don’t know where their 74 (7.9) 348 (15.9) < 0.001 children are in free time away from home (sometimes/seldom/ never) Environmental Travelled to school 0-15 minutes 708 (68.3) 1570 (63.5) 0.13 16-30 minutes 210 (18.7) 552 (23.9) ≥31 minutes 168 (13.0) 357 (12.6) Travelled from school 0-15 minutes 646 (61.4) 1202 (48.4) < 0.001 16-30 minutes 253 (22.7) 738 (30.6) ≥31 minutes 184 (16.0) 533 (20.9) Living in urban area (urban) 857 (84.7.0) 2075 (83.7) 0.86