def _ggm_prob_wait_whitt_z(ca2, cs2):
    """
    Equation 3.8 on p139 of Whitt (1993). Used in approximation for P(Wq > 0) in GI/G/c/inf queue.

    See Whitt, Ward. "Approximations for the GI/G/m queue"
    Production and Operations Management 2, 2 (Spring 1993): 114-161.


    Parameters
    ----------
    ca2 : float
        squared coefficient of variation for inter-arrival time distribution
    cs2 : float
        squared coefficient of variation for service time distribution

    Returns
    -------
    float
        approximation for intermediate term z (see Eq 3.6)

    """

    z = (ca2 + cs2) / (1.0 + cs2)

    return z