def null_scale(self):
        """
        Optimal s according to the marginal likelihood.

        The optimal s is given by

            s = (n·p)⁻¹𝐲ᵀK⁻¹(𝐲 - 𝐦),

        where 𝐦 = (A ⊗ X)vec(𝚩) and 𝚩 is optimal.

        Returns
        -------
        scale : float
            Optimal scale.
        """
        np = self._nsamples * self._ntraits
        b = vec(self.null_beta)
        mKiy = b.T @ self._MKiy
        sqrtdot = self._yKiy - mKiy
        scale = sqrtdot / np
        return scale
Optimal s according to the marginal likelihood.

        The optimal s is given by

            s = (n·p)⁻¹𝐲ᵀK⁻¹(𝐲 - 𝐦),

        where 𝐦 = (A ⊗ X)vec(𝚩) and 𝚩 is optimal.

        Returns
        -------
        scale : float
            Optimal scale.