def ratio(self, operand):
        """Calculate the ratio of this `Spectrogram` against a reference

        Parameters
        ----------
        operand : `str`, `FrequencySeries`, `Quantity`
            a `~gwpy.frequencyseries.FrequencySeries` or
            `~astropy.units.Quantity` to weight against, or one of

            - ``'mean'`` : weight against the mean of each spectrum
              in this Spectrogram
            - ``'median'`` : weight against the median of each spectrum
              in this Spectrogram

        Returns
        -------
        spectrogram : `Spectrogram`
            a new `Spectrogram`

        Raises
        ------
        ValueError
            if ``operand`` is given as a `str` that isn't supported
        """
        if isinstance(operand, string_types):
            if operand == 'mean':
                operand = self.mean(axis=0)
            elif operand == 'median':
                operand = self.median(axis=0)
            else:
                raise ValueError("operand %r unrecognised, please give a "
                                 "Quantity or one of: 'mean', 'median'"
                                 % operand)
        out = self / operand
        return out
Calculate the ratio of this `Spectrogram` against a reference

        Parameters
        ----------
        operand : `str`, `FrequencySeries`, `Quantity`
            a `~gwpy.frequencyseries.FrequencySeries` or
            `~astropy.units.Quantity` to weight against, or one of

            - ``'mean'`` : weight against the mean of each spectrum
              in this Spectrogram
            - ``'median'`` : weight against the median of each spectrum
              in this Spectrogram

        Returns
        -------
        spectrogram : `Spectrogram`
            a new `Spectrogram`

        Raises
        ------
        ValueError
            if ``operand`` is given as a `str` that isn't supported