How do control charts visualize quality variations over time?

Control charts are a fundamental tool in quality management, specifically designed to visualize variations in a process over time. They do this by applying upper and lower control limits, often derived from statistical measures such as standard deviation. These limits help to identify whether a process is in control, meaning that the variations are due to common causes and are expected. When the process performance falls outside of these control limits, it indicates the presence of special causes that require investigation.

By providing a graphical representation of these limits alongside the data points collected over time, control charts allow teams to monitor stability and consistency in process performance. This helps organizations respond effectively to variations, ensuring that they can make data-driven decisions aimed at quality improvement.

The other options do not adequately capture the primary function of control charts. Color coding may enhance the visual presentation but does not convey the statistical insights necessary for understanding variations. Representing frequency counts or illustrating customer satisfaction levels pertains to different aspects of data analysis that fall outside the specific purpose of control charts in monitoring process stability and variation.