BMC Public Health 2008, 8:171 http://www.biomedcentral.com/1471-2458/8/171 Education Parents were asked about their total years of education and the education of their current partner if applicable. Parental educational level (based on the parent with the most total years of schooling) was classified as less than 10 years, 10 years and more than 10 years. Living space > 75 m2 Parents were asked how many square metres (m2) the child's living space had. The median size was 75 m2 and a binary variable with values <= 75 m2 and > 75 m2 was derived. Number of persons living in the child's living place Parents were asked how many persons were living in the child's living place. The variable used for analysis had the following values: 1, 2, 3, 4, 5, 6, 7, 8, 9 where 9 indicates 9 and more persons. Single child Parents were asked how many older/younger siblings their child had. From this information, a variable "single child" was created. Attendance of day nursery or being otherwise looked after Parents were asked whether the child spent more than six hours per day at a place other than the living address. If so, it is assumed that the child attended a day nursery or was otherwise looked after. Smoking in the living place Parents were asked whether there was smoking in the place where the child was living at the time. Passive smoking during the first three years of life Parents were asked whether anybody smoked in the place where the child spent most of his/her time during the first three years of life. Smoking during pregnancy Parents were asked whether the mother of the child smoked during pregnancy. Breastfeeding for more than three months Parents were asked for how many weeks mothers breast- fed their children. From this information, a variable "breastfeeding for more than three months" was derived. Preterm delivery Parents were asked whether the birth of their child was premature. Birth weight Parents were asked about the birth weight of their chil- dren. The continuous variable was then transformed into the following categories: < 2500 g; 2500 g – < 3000 g; 3000 g – < 3500 g; 3500 g – < 4000 g; >= 4000 g. Year of study 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000. Study region The geographically defined areas were coded according to whether they were located in the East or in the West of Germany. Study location The geographically defined areas were coded according to whether they were located in a rural or an urban area. Statistical analysis Data analysis was performed with SAS® Version 9.1 WIN. Subjects were included only if there were no missing data on age, sex and BMI. Prevalence differences between groups were tested using chi squared tests. Trends across categories were analysed using Cochran-Armitage trend tests. Logistic regression was performed to obtain adjusted odds ratios (ORs) and 95% confidence intervals (95% CIs), measuring the association between exposure varia- bles and the odds of being overweight or obese, respec- tively, in univarate and in mutually adjusted models. Model fit was evaluated by performing Hosmer and Leme- show Goodness-of-Fit tests, testing the association of pre- dicted probabilities and observed responses [29]. Results The response rate for the questionnaire was 79% of those children who participated in the school entry examina- tions in the respective places and years. Table 1 shows the characteristics of the study population. The average BMI was 15.8 kg/m2. 15.5% of the participating children were classified as overweight and 4.3% as obese. The children's average age was 6.3 years with half of the study popula- tion being male. The large majority had a German nation- ality. Most children came from urban areas and from the Eastern study regions and had a birth weight between 2500 g and 4000 g. The level of parental education was comparably high with only 11.5% reporting less than 10 years of schooling. About half of the children lived in a place sized > 75 m2, about half attended day nursery and roughly every second child was exposed to environmental smoking at home. About one in three children was a sin- gle child, exposed to smoking during the first three years of life and was breastfed more than three months. Only a minority of children was delivered preterm or was exposed to maternal smoking during pregnancy. Page 3 of 10 (page number not for citation purposes)