def deriv(self, mu):
        """
        Derivative of the variance function v'(mu)
        """
        from statsmodels.tools.numdiff import approx_fprime_cs, approx_fprime
        # return approx_fprime_cs(mu, self)  # TODO fix breaks in `fabs
        # TODO: diag is workaround problem with numdiff for 1d
        return np.diag(approx_fprime(mu, self))