Which statements are true about control charts?

Control charts are a statistical tool used in quality control to monitor the performance of processes over time. The correct choice highlights the function of control charts in detecting nonstationarity, which is key for maintaining process stability.

Nonstationarity refers to changes in the statistical properties of a process over time, like shifts in the mean or variance. Control charts are designed to identify these shifts by plotting process data points and overlaying control limits—these limits help determine if a process is in control (stable) or out of control (unstable). If points fall outside these control limits or show a trend, it signals that the process may be nonstationary, prompting further investigation or corrective action.

Statements that are not true about control charts tend to mischaracterize their functions. For example, while control charts do relate to means, they are not primarily focused on estimating means but rather on monitoring the consistency of a process. Control charts typically have both an upper and lower control limit rather than a single reference limit. Additionally, while variance can be analyzed through control charts, they do not analyze variance solely; they evaluate both the mean and variability of the process data. Thus, the function of control charts encompasses a broader range than merely variance analysis.