Second derivative of the link function g''(p)

        implemented through numerical differentiation

def deriv2(self, p):
        """Second derivative of the link function g''(p)

        implemented through numerical differentiation
        """
        from statsmodels.tools.numdiff import approx_fprime_cs
        # TODO: workaround proplem with numdiff for 1d
        return np.diag(approx_fprime_cs(p, self.deriv))