Which statement is true regarding control charts?

Control charts are statistical tools used primarily in quality control to monitor the performance of processes over time. They help identify variations in processes and signal when a process may be out of control due to non-random variations.

When considering the statements about control charts, the affirmation that they are applicable only to quality metrics, while somewhat limiting, captures a significant aspect of their use. Control charts are designed to track how specific quality metrics change over time, ensuring that processes remain within desired thresholds.

Moreover, the time dimension is critical in using control charts. These charts plot data points over a specific timeframe, allowing for the analysis of trends, shifts, and cycles in the data. By focusing on how a process behaves over time, control charts help identify abnormal variations that could impact quality or performance.

Consequently, acknowledging both that control charts apply to quality metrics and that they inherently consider time reinforces the understanding of the dual focus of these tools. Thus, the belief that both statements are true leads to the conclusion that the collective statement "All of the above" is accurate, as it aligns with the fundamental principles of how control charts operate and what they are designed to measure.