A total of 1,517 children were identified through class rosters at the beginning of 9th grade. Those with a special classroom placement, and who spoke English as a second language were ineligible to participate. Based on this selection criteria 98% of the adolescents were eligible ( n = 1,487). Parental consent was obtained on 99% of the eligible adolescents ( n = 1,478), and 97% of the eligible participants completed the baseline survey ( n = 1,429). A total of 30 adolescents were absent on the assent/survey day, and a further 19 were not interested in the study. The participants who completed the baseline survey were followed-up every 6 months over a 4-year period. The University of Pennsylvania Institutional Review Board granted ethical approval for the study. The participants self-reported their height and weight, from which BMI (kg m −2) was calculated. It has been shown that self-reported and measured BMI's are highly correlated ( ( 8)). The height and weight data were compared to CDC growth charts, to help identify any excessive high and low self-reported values ( ( 9)). Time spent watching television/videos and playing video games, on a week night during the school-term, was self-reported by the participants (5 h day −1). The questions used to capture screen time have previously been validated ( ( 10)). Gender, race, and maternal education (marker for socioeconomic status) were included as covariates in the present study.
