Utility functions. (A) Probability of at least 25% analgesia minus the probability of at least 50% respiratory depression versus effect-site concentration. (B) Probability of at least 50% analgesia minus the probability of at least 50% respiratory depression versus effect-site concentration. (C) Probability of at least 25% analgesia minus the probability of at least 50% respiratory depression versus time after a bolus infusion of 50 µg alfentanil. (D) Probability of at least 50% analgesia minus the probability of at least 50% respiratory depression versus time after a bolus infusion of 50 µg alfentanil. The continuous lines represent the utility function, the broken lines the 95% CIs.