The staff in the outpatient department complete the morning schedule at varied times.

Explanation: Common-cause variation refers to inherent, random fluctuations within a process. According to NAHQ CPHQ study materials, a control chart is the most effective tool for identifying common-cause variation in health data analytics. It plots data over time against control limits, distinguishing between common-cause and special-cause variations. Pie charts (A), bar charts (B), and line graphs (C) are not suited for variation analysis. NAHQ emphasizes control charts for process stability monitoring.