Derivative of the inverse of the power transform

        Parameters
        ----------
        z : array-like
            `z` is usually the linear predictor for a GLM or GEE model.

        Returns
        -------
        g^(-1)'(z) : array
            The value of the derivative of the inverse of the power transform
        function

def inverse_deriv(self, z):
        """
        Derivative of the inverse of the power transform

        Parameters
        ----------
        z : array-like
            `z` is usually the linear predictor for a GLM or GEE model.

        Returns
        -------
        g^(-1)'(z) : array
            The value of the derivative of the inverse of the power transform
        function
        """
        return np.power(z, (1 - self.power)/self.power) / self.power