def _opposite_axis_margin(self):
        """ndarray representing margin along the axis opposite of self._axis

        In the process of calculating p-values for the column significance testing we
        need both the margin along the primary axis and the percentage margin along
        the opposite axis.
        """
        off_axis = 1 - self._axis
        return self._slice.margin(axis=off_axis, include_mr_cat=self._include_mr_cat)