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Using statistical process control to demonstrate the effect of operational interventions on quality indicators in the emergency department.

When our emergency department (ED) initiated a continuous quality improvement (CQI) program, we selected as a quality indicator the percentage of patients leaving without being seen (LWBS) by a physician. Because the primary reason for LWBS patients was determined to be dissatisfaction with waiting time, we devised four interventions in clinical operations to decrease delays in patient flow through the ED. Statistical process control (SPC) methodology was then used to assess the effect of these interventions. Because baseline data were available, we constructed control charts of the percentage of LWBS patients versus consecutive months beginning in January 1990 with the mean percentage of LWBS patients and upper and lower control limits. Postintervention data, plotted using control statistics from the baseline period, demonstrated sustained special-cause variation, indicating a fundamental change in the overall system. A new control chart was then constructed using postintervention data. A significantly lowered mean percentage LWBS and a narrowed control limit range were observed, leading to the conclusion that the interventions improved the quality of care as measured by a reduction in percentage LWBS.