BMC Public Health 2009, 9:485 http://www.biomedcentral.com/1471-2458/9/485 7-10, 'high':11-16. The 1 week test-retest of the ownership of these amenities showed consistent answers ranging between 86.1% and 99.1%. rates or in both of them combined were included in the regression models. Colinearity and plausible interactions between variables in the model were investigated. The present household economic situation index was constructed based on 4 questions from the parents' ques- tionnaire using a 5-point Likert Scale where 1 represented the best situation and 5 the worst. These questions were: "Do you have enough money to cover family needs now?", "At the present time, do you have any debts?", "At the present time, to what extent do you have to postpone paying bills?", and "During the past school year, did you have to borrow money to cover family needs?" A total index was formulated by adding up these values. Three categories were constructed: 'worse off': 14-20; 'medium': 7-13; 'better off': < 7. Spearman correlation between this index and STL was 0.42 (p < 0.001). Food availability was assessed based on the following question from the parents' questionnaire: "Is there enough food for all family members on daily basis now?" Food was considered available if the family had enough food for all members "most of the time" or "always." Onset of puberty Onset of puberty was assessed by the question "have you reached puberty i.e. "have you had your first period?" for girls and "have you noticed deepening of your voice?" for boys. The 1 week test-retest for the onset of puberty showed consist answers in 92.3% in boys and 95.2% in girls. Statistical analysis EPI 2002 (Center for Disease Control and Prevention, Atlanta, Georgia) was used to calculate BMI, BMI and height percentiles. All other statistical analyses were done using Stata 10.0 (Stata Corporation, College Station, TX) and were adjusted for cluster sampling design. The sample was weighed according to sample and population size (inverse of sampling probability) in each grade and in each governorate. χ2 tests were used to compare frequen- cies and t-tests were used to compare means. A signifi- cance level of 0.05 was used for all statistical analyses. Separate analyses were run for boys and girls in each gov- ernorate. Multivariate logistic regression analysis was used to model the association between the outcome variables (overweight/obesity), (underweight) and (stunting) and selected sociodemographic characteristics. As no interac- tions were found between region (Ramallah/Hebron) and the rest of the independent variables in any of the models, regression analysis was done for both regions together. Age, region and variables with p < 0.05 in the bivariate analysis tables for each gender in any of the two governo- Ethical considerations Informed consent was obtained from parents, students and school principals with standard assurances of confi- dentiality. They were informed that answering the anony- mous questionnaire was voluntary. Moreover, the class teacher was not present while administering the question- naires to assure confidentiality. The study was a collabora- tion between the Institute of General Practice and Community Medicine at the University of Oslo and the Institute of Community and Public Health at Birzeit Uni- versity. The protocol was approved by both institutes. The study was also approved by the Palestinian Ministry of Education and Higher Education, by UNRWA Office of Education and by Regional Ethical Committee of Norway. Results Sample characteristics and response rates Two schools in Ramallah governorate refused to partici- pate in the study and were replaced by schools from the same stratum of the sampling frame. In total, 2170 stu- dents in 65 classes were invited to participate, while 2032 (93.6%) actually participated in the study. Reasons for non-participation included: absence on the day of the sur- vey (71) and refusal from parents or students or both (67). The analysis presented in this paper excluded 90 stu- dents being younger or older than the defined age groups of 13, 14 and 15 years. The final sample thus consisted of 1942 students (95.6% of those participating). In total, 1863 (95.9%) parents completed their questionnaires. No significant differences were found between responding and non-responding parents (n = 79) in terms of STL, res- idence (urban versus. rural), age groups or sex of students. However, 75.9% of non-responding parents were from Hebron (p < 0.05). Table 1 shows that girls were slightly over-represented in the sample and that the fathers of the students were more likely to have a higher education than the mothers in both governorates. The proportion of 8th grade students in the sample of Ramallah governorate was higher than that in the population, but that was not the case in Hebron (data not shown). More adolescents were in the 13 and 14 years age groups compared to the number in the 15 years age group in both governorates. In Hebron, mean family size was higher and mean STL was lower compared to Ramal- lah. Adolescents in Hebron were more likely to be living in urban areas (67.8% in Hebron versus 41.0% in Ramal- lah). Hebron had a higher proportion of students coming from households with worse off present household eco- nomic situation index than Ramallah. Page 4 of 12 (page number not for citation purposes)