Fig. 2. The mean monthly productivity (hours billed per attending anesthesiologist per clinical day) is indicated by the shaded area and left axis. The mean number of cases supervised concurrently by attending anesthesiologists is represented by the broken black line and right axis. A solid line at the level corresponding to two cases supervised concurrently (read from right axis) is provided for reference. All months are illustrated but not labeled. Concurrency was strongly correlated with productivity (r = 0.838, P < 0.001).