'Derivative of the inverse of the negative binomial transform
Parameters
z : array-like
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 negative
binomial link'

def inverse_deriv(self, z):

    t = np.exp(z)
    return (t / (self.alpha * ((1 - t) ** 2)))
