Fig. 2. Predicted (  lines ) and observed (  symbols ) dose–response relation of bupivacaine, levobupivacaine, and ropivacaine in 450 laboring parturients. Dose–response curves have been constructed using a probit regression model. Likelihood ratio tests have been used to compare the dose–response curves among the three groups. No significant difference (  P = 0.52) in the dose–response curves of levobupivacaine and ropivacaine were observed. Significant differences in the dose–response curves of bupivacaine and ropivacaine (  P = 0.002) and the dose–response curves of bupivacaine and levobupivacaine (  P = 0.0036) were noted. Definition of effective analgesia: The block was deemed successful if the visual analog scale score was less than 25 mm at 15 min after initiation of analgesia. The block was deemed unsuccessful if the visual analog scale score was greater than 25 mm at 15 min after initiation of analgesia.