Fig. 3. The tick-mark plot shows the response–no response (hemodynamic) versus plasma fentanyl concentrations. Each mark indicates the plasma fentanyl concentration and response to surgical incision. From these data, the predicted probability of no response (hemodynamic) plasma fentanyl concentration obtained using logistic regression is plotted in the upper graph. The bar indicates ± 1 SD of the estimate of the plasma fentanyl concentration producing a 50% probability of no hemodynamic response at surgical incision.