Study population and data collection
In 2005, baseline questionnaires were mailed out to about 200,000 adult students enrolled at Sukhothai Thammathirat Open University (STOU) and 87,134 (44%) aged 15 to 87 years responded. Details on cohort enrolment and overall methodology have been reported elsewhere . Responding students were similar to other STOU students for age, sex, marital status, income, courses of study and geographical location. Furthermore, the cohort participants were generally similar to the population of Thailand, especially in the 20-39 years age group, for sex ratio, geographical location, and socioeconomic status measured by median income . Participation in the study was voluntary and study leaders reassured participants that their personal responses were confidential. Cohort members were motivated by being fully informed about the purposes of the Thai Health-Risk Transition study and that they could contribute to public health knowledge in Thailand. A periodic newsletter provides information back to cohort members on study progress and follow-up continues .
The 2005 20-page baseline questionnaire covered a wide range of topics including demographic, socioeconomic and geographic attributes, health status including self-reported number of teeth, health service use, risk behaviors, injuries, dietary intake, physical activities, and family background. The questionnaire was developed by a multi-disciplinary team of experts in both Thailand and Australia. As far as possible we used standardized validated measures, including those used by the Thai National Statistical Office (e.g. income categories, geographical location, and household assets). There were many iterations and extensive pretesting of back translated Thai versions to ensure face and content validity. Pilot testing preceded the final version.
Here we analyse self-reported number of teeth and relate that to sex; age (3 groups: 15-29 years, 30-49 years, and 50 years and above), monthly income in Thai Baht (≤3000; 3001-7000; 7001-10000; and >10000; 40 Baht~$1US in 2005), education (highschool, post high school diploma/certificate, university or higher), and household assets (later categorized by total replacement value in Thai Baht into three groups: <30,000; 30,001-60,000; and >60,000). The household assets included general domestic items such as a microwave oven, electric fan, air conditioner, computer, radio, video/vcd recorder, washing machine, water heater, and telephone).
As well, we determined lifecourse urbanization based on geographic residence now (as an adult member of the cohort) and when aged 12 years--creating the following urbanization categories ('lifetime rural residents'; 'rural-urban migrants'; and 'lifetime urban residents'). The small number of cohort members (4%) who were categorized as 'urban-rural migrants' were excluded from the analysis so we could concentrate on the main categories that characterized the Thai population today.
Health-risk behavior variables included current regular tobacco smoking (never smoker, ex-smoker, regular smoker), alcohol drinking (regular or not), and soft drink intake (at least daily, or not). Childhood covariates were history of being breastfed as a child recalled by inquiring from relatives (yes, no, or do not know), father's education, and mother's education.