def _get_phi(self, imt, mag):
        """
        Returns the within-event standard deviation (phi)
        """
        phi = get_phi_ss(imt, mag, self.PHI_SS)
        if self.ergodic:
            C = self.PHI_S2SS[imt]
            phi = np.sqrt(phi ** 2. + C["phi_s2ss"] ** 2.)
        return phi
Returns the within-event standard deviation (phi)