def is_medium_truck(df):
    """Return if vehicle is a medium truck, per NHTSA convention."""
    yr = df['YEAR']
    body = df['BODY_TYP']
    return yr.between(1975, 1981) & body.isin([53, 54, 56]) | yr.between(
        1982, 1990) & body.isin([70, 71, 75, 78]) | (1991 <= yr) & body.isin([
        60, 61, 62, 64, 67, 71])
