def is_pickup(df):
    """Return if vehicle is a pickup truck, per NHTSA convention."""
    yr = df['YEAR']
    body = df['BODY_TYP']

    return ((yr.between(1975, 1981) & (body == 50)) |
            (yr.between(1982, 1990) & body.isin([50, 51])) |
            ((1991 <= yr) & body.between(30, 39)))