Surrogate indicators of socioeconomic status were parent education (categorized as having a 4-year baccalaureate degree or more, or less than a 4-year degree) and poverty index (percent of homes with incomes below the federal poverty level) for the census tract in which the family's home was located.[13] Weight was measured in duplicate to the nearest 0.1 kg using an electronic scale (Seca, Model 770; Hamburg, Germany), and height was measured in duplicate to the nearest 0.1 cm using a portable stadiometer (Shorr Productions; Olney, MD), after children removed their shoes and outer clothing. Body mass index (BMI) was calculated (kg/m2) from the averages of height and weight.