Fig. 2. Remifentanil–sevoflurane interaction for sedation. The Logit response surface model prediction for sedation for unstimulated volunteers is presented in A . An Observer's Assessment of Alertness/Sedation (OAA/S) score of 1 or less represents a sedated volunteer. A 0 indicates an OAA/S score of 2 or greater, and a 1 indicates an OAA/S score of 1 or less. The symbols show measured responses, and the surface predicted by the model is represented by the grid-lined surface . The raw data used to create this model are shaded based on the residual error. A topographic view of the 50% and 95% effect isoboles for probability of being sedated is presented in B . The OAA/S score at each target concentration pair is overlaid.