Fig. 4. Nonlinear regression analysis between performance error with Shafer model (PE-Shafer) and total body weight (TBW) for a random sample of approximately one half of the patients, termed the “training group.” The equation for the relationship is: PE(%) = 227.7 × e −0.026 kg− 55.69 (R squared = 0.662, P < 0.001). This equation was used to assess the improvement in the accuracy for prediction of plasma fentanyl concentrations in the remaining patients (the “testing group”).