Sampling methods for each participating country
| Country | Sampling methods |
| Brazil | Primary and secondary schools will be selected from high and low SES regions across different areas of the country. Additionally, schools situated in coastal and non-coastal cities will also be included. We will include private and public schools. |
| Canada | Primary and secondary schools will be selected in urban and rural areas within the regions of Lethbridge, Alberta (population ~100 000) and Ottawa, Ontario (population ~1 000 000). We will include public and catholic schools in both regions as well as French schools in Ottawa. Schools located in higher and lower income neighbourhoods relative to the median household income of their census metropolitan area will be invited. |
| China | Primary and secondary schools will be selected in urban and rural areas in the Shanghai metropolitan area (population ~24.9 million). We will include public and private schools from two districts, representing urban and rural areas, respectively. A multistage stratified cluster sampling method will be used. |
| Colombia | Primary and secondary schools will be selected in low, middle and high economic areas within urban areas and low-income in rural areas in Bogotá Colombia (overall population ~7 200 000 including ~2 000 000 children and adolescents). Bogotá is the capital city of Colombia and the largest city with urban and rural areas. We will include a convenience, but stratified sample of public or private schools. Schools will be selected based on SES and urban vs rural location areas. In Colombia, schools in rural areas are all low-income. |
| India | Students from schools in the city of Pune, India (population ~3.9 million) will be invited to participate. Both government-funded and private schools located in Pune (urban centre) and rural jurisdictions ~60 km from Pune will be included to ensure recruitment from across SES groups. Stratified random sampling will be used to recruit an equal proportion of males and females across different grade levels. Out-of-school adolescents will be recruited via a random sampling of households with children or adolescents. These households will be identified from an existing registry. Additional sampling from local community centres and temples will ensure the recruitment of children and adolescents across age groups. |
| Malawi | Primary and secondary schools will be selected in the urban and rural areas of Blantyre (total population ~1 300 000) and Zomba (total population ~900 000) districts in Southern Malawi. We will include public, private, and religious schools in the two districts. Out-of-school adolescents will be identified through (1) school records and youth clubs community-based groups within the settings located within the catchment area of the selected schools; and (2) households with under 18s in selected communities surrounding the sampled schools. Households with under 18s will be identified through existing registries maintained by community leaders. |
| Mexico | Primary and secondary public and private schools will be selected in urban and rural areas within the central, north and south regions of Mexico, to ensure the inclusion of diverse populations. In Mexico schools in rural areas are all low-income. We will also recruit children through sports centres, community-based and faith-based groups. |
| Nepal | Schools (representing public and private) of urban and rural areas from three ecological regions of the country – Mountain (Dolkha), Hills (Syangja) and Terai/Plains (Repandehi and Bardiya)—will be selected purposively. Schools of varying SES will be identified with the support of the provincial government and/or the local government. A convenient but stratified sampling method will be used to recruit participants. Out-of-school children and adolescents will be identified from the records of local government, youth clubs and/or the schoolteachers in specific study sites. |
| New Zealand | Primary, intermediate and secondary schools within the greater regions of Tāmaki Makaurau Auckland and Te Whanganui-a-Tara Wellington (the two largest cities by population in Aotearoa New Zealand), will be sent information sheets and assents/consents to participate. From these schools, we will obtain consent and assent from students and from these potential participants recruit 500 children and adolescents stratified by age, ethnicity, sex, SES and urbanisation. |
| Nigeria | A convenient but stratified sampling method will be used to recruit participants into the study. Participants will be selected from primary and secondary schools (balanced by public and private schools and area-level SES) in the rural and urban areas of Osun State in South West Nigeria. Area-level SES will be determined by information from the National Population Commission and/or expert judgement using information about property values and aesthetics and building quality in the neighbourhoods. |
| Spain | Primary and secondary schools will be selected in urban and rural areas within the regions of Madrid (population ~6 700 000), Andalusia (population ~8 500 000), Basque Country (population ~2 200 000) and Extremadura (population ~1 000 000). We will include public and private schools in all regions. Schools located in higher and lower income neighbourhoods relative to the median household income of their census metropolitan area will be invited. |
| Sweden | Primary and secondary schools will be selected in urban and rural areas within Sweden (ie, Stockholm area, medium and smaller cities). The schools will be selected to represent different SES areas. |
| Thailand | Primary and secondary schools will be selected from urban (Bangkok, population 5 588 222) and rural areas (Nakhon Pathom province, population 920 729) of Thailand. To accommodate for the variance between schools, the sample will be stratified proportionately based on school size. Within the schools, balanced samples will be drawn with respect to sex and grades. |
| United Arab Emirates (UAE) | Dubai Emirates has the largest and most diverse population in the UAE with school-aged children in private and public (governmental) primary and secondary (high) schools. Public schools are sex-segregated and primarily include UAE nationals. Most private schools are co-educational (a mix of expatriate and UAE national children). A multistage stratified random sampling technique will be used to ensure that the overall sample is representative of primary and secondary school children attending both private and public schools in Dubai. During Stage 1, private and public schools will be split into two groups. Private schools will be divided into five strata based on the predominant nationality of students attending the school (eg, European/North American, Emirati, Middle Eastern, South Asian and South-East Asian) and public (governmental) schools will be split by sex and school cycle (ie, cycle 2 or 3). Next, private schools will be stratified using annual school fees as a proxy of SES. Data on student nationality and annual school fees is publicly available. The total sample size will be allocated proportionately to the population size of children in each stratum to yield a representative and proportionate sample. Once a school is randomly selected, a list of classrooms will be compiled for each year group in the school. A simple random sample of classrooms will be selected from each school to achieve the proportionate sample. The consenting students in the selected classrooms will form the final sample. |
SES, socioeconomic status.