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)
