Teun Remmers

88 | Chapter 6 one season; (9, 12, 21). Single measurement periods may lack variability in weather elements, and may not reflect yearly (season-related) variability of weather elements (9). Studies measuring PA within a wide timespan using multiple measurement periods across seasons are needed to investigate the full range of these weather elements. In addition, the three studies described above focused on explaining variability between children (i.e. inter-individual variability) (9, 12, 21), but PA also shows considerable day-to-day variation within children (i.e. intra-individual variability) (12). As the influence of weather on children’s PA patterns is a complex interplay between inter- and intra-individual relationships (24), understanding these relationships requires both knowledge from intra- individual, as well as inter-individual mechanisms. Finally, it is also of interest to see whether different effects may occur in population subgroups. For example, overweight or obese children may experience physical complaints or insecurity as a more proximal barrier to perform PA (irrespective of more distal weather elements) (8). The present study first aims to examine intra-individual associations between daily weather elements and children’s PA across a one-year timespan that reflects variability of weather across four seasons. Our second aim was to examine whether the influence of weather elements differs by day of the week (weekdays vs. weekends), gender, age, and Body Mass Index (BMI). Methods Procedures and participants Primary schools located within a 40km radius of the Melbourne Central Business District, Australia, and with enrolments of greater than 200 pupils were identified and stratified into tertiles of socioeconomic status (SES) based on postcode using the Socio-Economic Index for Areas (1). In 2012, schools were randomly selected from each SES strata and invited to participate in the Patterns of Habitual Activity across Seasons (PHASE) study, which used a repeated measures design to examine seasonal changes in children’s activity levels (26). Nine schools (one low, three medium, and five high SES) agreed to participate in the study and the school Principal provided written informed consent. Ethical approval for the study was provided by Deakin University Human Ethics Advisory Group Committee (HEAG), the Department of Education and Early Childhood Development, and the Catholic Education Office (Melbourne). All children in Years 4 and 5 (aged 8-11 years) were invited to participate in the study (n=1270). Parental written informed consent was provided for 326 children (162 boys, 164 girls; 25.7% response rate) to participate in at least one component of the study at baseline (i.e. accelerometers, inclinometers, survey, anthropometry, focus groups). No information was obtained about non-responders as it is an ethical requirement in Australia for active informed consent to be provided. Data were collected from children during four school terms that closely correspond with each season in south-east Australia: Winter (August-September 2012; Term 3); Spring (October-November 2012; Term 4);