'Derivative of the inverse of the logit 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 logit function'

def inverse_deriv(self, z):

    t = np.exp(z)
    return (t / ((1 + t) ** 2))
