code: def beta(self):
          
          from numpy_sugar.linalg import rsolve
  
          return rsolve(self._X["VT"], rsolve(self._X["tX"], self.mean()))
docs: Fixed-effect sizes.
  
          Returns
          -------
          effect-sizes : numpy.ndarray
              Optimal fixed-effect sizes.
  
          Notes
          -----
          Setting the derivative of log(p(𝐲)) over effect sizes equal
          to zero leads to solutions 𝜷 from equation ::
  
              (QᵀX)ᵀD⁻¹(QᵀX)𝜷 = (QᵀX)ᵀD⁻¹(Qᵀ𝐲).