Wong et al. BMC Public Health 2011, 11:618 http://www.biomedcentral.com/1471-2458/11/618 population being studied. There have been at least four studies on AST among Canadian children and youth [5-8]. In terms of socio-demographic factors, girls [5,7,8], students with lower socio-economic status [9], and older students [7-9] were consistently found to be less likely to walk or cycle to school. Only one Canadian study has examined psychological correlates of AST, finding no association between per- ceived athletic ability, perceived parent encouragement, and perceived weight status and AST [8]. Of growing interest is the concept of childhood independent mobi- lity. Independent mobility is defined as the ‘opportunity for children to move around independently without adults as escorts’ [10]. Independent mobility is of inter- est because children with greater independent mobility may spend more time outdoors being physically active [11]. Autonomy with respect to mobility decisions and outcomes has been found to be an important psycholo- gical factor impacting on school transport [11-16]. For example, youth who reported their parents knew little or nothing about their whereabouts after school were more likely to actively commute to school [13]. Notably, the impact of whether children and youth are involved in the process of decision-making (which may reflect both independence and motivation for walking) on school transport mode has not been studied. For the behavioural correlates of active school trans- port, children and youth who actively travelled to school were consistently found to be more physically active regardless of age, gender, and country [8]. Environmental factors also may play an important role in school trans- port. Canadian studies have found that youth living in urban areas [7,8] and within a shorter distance to school were more likely to actively commute to school [5,6]. These findings are also consistent with international research [3,17]. Moreover, regional differences in active school transport have been consistently reported [18,19]. More broadly, within the school transport literature internationally, separate analyses for school mode choice in the morning and afternoon has not been a common practice. For example, in the comprehensive review by Sirard and Slater [3], this distinction is not explicitly dis- cussed. In a recent systematic review on the built envir- onment and active school travel, only five out of 14 studies analysed school mode choice in the a.m. and p. m. periods separately [20]. Recent research highlights the importance of making such a distinction [6]. Impor- tantly, no study has examined the correlates of the most common mode shift - shifting from passive modes of travel in the morning to active modes of travel in the afternoon [3,20]. It is important to identify these mode shifters since promoting active school transport may be more feasible for these individuals than those being dri- ven both ways. These parents or other caregivers already Page 2 of 12 allow their children to take at least one trip actively. Identifying who these students are may also be helpful in the consideration of interventions that specifically tar- get the a.m. or p.m. period. Apart from some exceptions [21], the prevalence and correlates of both elementary and secondary school students have not commonly been analysed separately. Existing studies [5,7,8] have also relied on convenience samples for their analyses. To address these gaps, the present study uses a provincially representative sample of elementary and secondary school students in Ontario, Canada to examine the fol- lowing two questions: i) what is the prevalence, and the correlates of school travel mode to school and back home and ii) what are the correlates associated with the school travel choices of students who shift from passive to active modes between the a.m. and p.m. school travel time periods. Methods Subjects and procedures Three thousand and six hundred students in grades 7 to 12 in Ontario completed the 2009 cycle of the Ontario Student Drug Use and Health Survey (OSDUHS), a self- administered, anonymous questionnaire, during a class period. The OSDUHS is a repeated cross-sectional sur- vey conducted every two years by the Centre for Addic- tion and Mental Health, Toronto, Canada, to assess the health behaviours of youth in Ontario. These respon- dents were selected using a stratified two-stage cluster sampling method that used representative sampling of all grade 7 to 12 students in publicly funded schools in Ontario. Stratified by school type (elementary and sec- ondary school) and by region (n = 4; Toronto; Northern Ontario [Parry Sound District, Nipissing District and farther north]; Western Ontario [Hamilton, Peel District, Dufferin County and farther west]; and Eastern Ontario [Ottawa, Leeds-Grenville-Lanark District, Haliburton- Kawartha-Pine Ridge District, Simcoe County, Durham Region, and York Region]), schools were randomly selected first, followed by a random selection of classes of each grade within each selected school. Of the 14,196 students enrolled in selected classes, 9,241 participated in the survey. The student participation rate was 65% (13% absenteeism and 22% unreturned consent forms or parents’ refusal). One hundred and twenty-nine respon- dents were dropped from the dataset, due to failure to report valid responses for age or sex, or because they did not complete at least half of the questionnaire (9,112 included). Of these 9,112 cases, 1,533 elementary students (Grade 7 and 8) and 2,728 secondary (Grade 9 to 12) completed Form B questionnaires that included questions about school transport developed by the authors. One hundred fifty-seven elementary school stu- dents did not complete at least half of the school