R-dihydroetorphine response surface; continuum of probabilities of R-dihydroetorphine-induced analgesia and respiratory depression as function of R-dihydroetorphine effect-site concentration (A and B). The color shading (green to yellow and red to orange) represents the context dependency of the utility functions on the postulated threshold for analgesia. The iso-utility lines in B represent the border in between areas of at least 50% analgesia (A++), at least 25% analgesia (A+), no analgesia (A−), at least 50% respiratory depression (R+), and no respiratory depression (R−). At 20 pg/ml, the probability of analgesia without any respiratory depression (green surface) is about 40% as depicted by the dotted line (A+/R−), while at 60 pg/ml and higher concentrations, this same probability remains steady at 20%. This then indicates that at 20 pg/ml, the probability of respiratory depression (irrespective of the presence of analgesia) is 60%, while at concentration greater than 60 pg/ml, this probability remains steady at 80% (as depicted by the area above the dotted line A+/R−).