Scheduling surgical cases and adhering to the schedule during the course of the day are complicated by the fact that the demand for surgery is often unpredictable and the length of the surgery itself varies. To better manage the surgical schedule, use control charts to plot data over time to study the variation in case length. The control chart provides estimates of the variation that must be taken into account in scheduling. A control chart will identify the normal variation in the system (common cause variation), as well as variation that is due to unusual or unpredictable cases (special cause variation).

Special causes of variation in surgery may be related to routine cases that develop unpredictable complications, unexpected shortages of staff, last-minute changes in physician schedules, and unavailable or malfunctioning equipment. These special causes of delay are not predictable, but can be eliminated or minimized by building contingencies into the surgical system to reduce their impact on the system.