A previously developed linear-spline model,5
with
knots placed at ages 12, 14, and 16 years, was
used to capture the nonlinear pattern of growth
during adolescence (see the Supplementary Appendix
for details). The model included adjustments
for sex, race, Hispanic ethnic background,
height, height squared, body-mass index (BMI,
the weight in kilograms divided by the square of
the height in meters), BMI squared, and presence
or absence of respiratory-tract illness on the day
of the pulmonary-function test