Figure 4. Mean 24-h intervention total for each group (mean +/− SD). Need for interventions varied significantly between groups (analysis of variance, P = 0.0004). Dotted line represents trend toward increasing need for interventions with increasing dose (linear regression analysis, P = 0.0002); the model predicts the number of interventions necessary will increase by 0.5 (95% CI, 0.22-0.75) for each increase in intrathecal morphine dose of 0.1 mg.