def _get_stddevs(self, C, mag, stddev_types, num_sites):
        """
        Return standard deviation as defined in eq.11 page 319.
        """
        std = C['c16'] + np.zeros(num_sites)

        if mag < 7.4:
            std -= 0.07 * mag
        else:
            std -= 0.518

        # only the 'total' standard deviation is supported, therefore the
        # std is always the same for all types
        stddevs = [std for _ in stddev_types]

        return stddevs
Return standard deviation as defined in eq.11 page 319.