def bandwidth_factor(nbr_data_pts, deriv_order=0):
    """
        Scale factor for one-dimensional plug-in bandwidth selection.
    """
    if deriv_order == 0:
        return (3.0 * nbr_data_pts / 4) ** (-1.0 / 5)
    if deriv_order == 2:
        return (7.0 * nbr_data_pts / 4) ** (-1.0 / 9)
    raise ValueError('Not implemented for derivative of order {}'.format(
        deriv_order))
