BMC Public Health 2009, 9:485 http://www.biomedcentral.com/1471-2458/9/485 A sample size of 1000 students in each governorate was set to have a maximum margin of error of 2.9% in the prevalence estimates of overweight/obesity using the fol- lowing formula: E = 1.96 DE * (p(1 − p)) / n where p is the proportion of overweight/obesity (in this case 18%, based on a similar study from the North Gaza strip) [16]; DE is the design effect that is estimated at 1.5 based on previous studies in schools; the value 1.96, assures a confidence level of 95%; and E is the expected maximum margin of error. As the average class size was about 31 students in Ramal- lah and 34 students in Hebron, 65 classes were selected: 34 in Ramallah and 31 in Hebron. All students in the cho- sen classes were invited to participate in the study. Data Collection The cross-sectional survey was conducted between March and May 2005. Self administered questionnaires were used to collect information from students and parents. The researcher and trained field workers gave students standardized instructions on filling in the questionnaires, which were completed in the classroom in 1.5-2 hours. Field workers were present during administering the ques- tionnaires to clarify questions when needed. The students' questionnaire included questions about age, residence, household amenities and onset of puberty. Seven core questions in the students' questionnaire were adapted from the World Health Organization (WHO)-question- naire on Health Behaviour in School-aged children (HBSC) [17]; these were: month and year of birth, onset of puberty, ownership of car, computer and Internet con- nection at home. The parents' questionnaire included household information such as family size, parents' edu- cation, and indicators of economic situation such as the ability to pay bills, cover family needs and having any debts. The parents' questionnaire was taken home by stu- dents and returned to school the following day. The stu- dents' and parents' questionnaires were piloted and adjusted before the survey. The students' questionnaire was tested for reliability (one week test-retest) on a differ- ent sample of 115 students in the same age group. Measures Anthropometric measures Students' weights were measured in light clothes without shoes to the nearest 0.1 kg using a portable scale (Seca 780/783, Hamburg, Germany). The scales were calibrated daily at the study site with a known weight. Heights were measured to the nearest 0.1 cm using a portable stadiom- eter (Seca 220) mounted on the scale. Students stood up straight in bare feet, with heels, buttocks and back touch- ing the stadiometer. Measurements were taken once and recorded by one trained field worker during the early morning classes before the school recess for all students. Intensive training was conducted before the field work to ensure reliability of the measurements and decrease inter- and intra field workers' errors. BMI was calculated as weight in kilograms divided by height in meters squared. For the purpose of regional comparisons, BMI was catego- rized based on age-and sex-specific cut-off values of the 2000 Centers for Disease Control and Prevention (CDC) growth charts. The categories were underweight (< 5th percentile), normal weight (5th to 85th percentile), over- weight (85th to 95th percentile), and obese (> 95th per- centile) [18]. For the purpose of international comparisons, BMI was also categorized using the IOTF criteria. Overweight and obesity were defined as BMI cut offs corresponding to adult cut-offs of overweight and obesity (BMI of 25 and 30 kg/m2, respectively) [19]. Stunting was defined as height-for-age below the 3rd per- centile [18]. Sociodemographic factors Age of the students was obtained from the date of birth as reported by the students. Three age groups were con- structed: age group 13: 13 years 0 months to 13 years 12 months, age group 14: 14 years 0 months to 14 years 12 months, and age group 15: 15 years 0 months to 15 years 12 months. Urban/rural classification was based on the Palestinian Central Bureau of Statistics (PCBS) classification which is based on services and population size [13]. Father's educational level was divided into 3 categories as follows: "Low": illiteracy, less than secondary school education, "Medium": secondary school education" and "High": college, university or higher. Mother's educational level was constructed in the same way as the father's, but because of few subjects in the high educational category, the last two categories were col- lapsed to one, that is, medium/high. The household standard of living (STL) index was based on household possessions. Ownership of central heating, family car, family mobile phone, personal mobile phone, indoor bathroom, water pipes, refrigerator, full automatic washing machine, color TV, satellite, video, computer, dish washer, microwave, vacuum cleaner and internet connection were summed; each item was given a value of 1. Three categories were constructed: 'low': 0-6, 'medium': Page 3 of 12 (page number not for citation purposes)