The data can find in potuse (faraway package).
The national Youth Survey collected a sample of 11-17 year-olds
with 117 boys and 120 girls, asking questions about marijuana
usage. This data is actually longitudinal – the same boys and girls
are followed for five years. However, for the purposes of this
question, imagine that the data is cross-sectional, that is, a
different sample of boys and girls are sampled each year. Build a
model for the different levels of marijuana...