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])))