Which statement about control charts is incorrect?

Control charts are a vital tool in quality management, used to monitor processes over time and identify variations that may indicate potential issues. The primary purpose of a control chart is to illustrate the stability of a process and ensure that it operates within established control limits.

The correct answer indicates that the statement about upper control limits being always ± 1.8 standard deviations is incorrect. In standard practice, control limits on a control chart—specifically the upper control limit (UCL) and lower control limit (LCL)—are typically set at ± 3 standard deviations from the process mean. This is based on the properties of the normal distribution, where approximately 99.73% of the data points fall within three standard deviations. Setting limits at 1.8 standard deviations would significantly increase the likelihood of false positives, where a process appears out of control even when it may not be.

The other statements reflect understanding of control charts. It is common for control limits to be ± 2 standard deviations or more likely ± 3 standard deviations from the mean. Additionally, control charts serve the crucial function of measuring key processes over time, allowing organizations to visualize performance and identify trends or anomalies effectively. Thus, the choice about upper control limits using 1.