Fig. 2. Correlation of hourly billing productivity and surgical duration for all groups (N = 20). Hourly billing productivity (tASA/h) highly correlates (r =−0.85) with surgical duration (hours billed per case). Differences in tASA/h results from differences in base units billed per hour (base/h). Differences in base/h can be caused by base/case or the number of cases performed per hour. The number of cases per hour is indirectly related to surgical duration. tASA = total ASA units billed; 1 h = 4 time units; case = case billed.