Which of the following elements is not found on an X-bar and R control chart?

An X-bar and R control chart is a tool used in statistical process control to monitor the mean and variability of a process over time. The X-bar chart specifically tracks the sample means of subgroups, while the R chart monitors the range within those subgroups.

The capability ratio, often represented as Cp or Cpk, is not part of the X-bar and R control chart. Instead, it's a statistical measure used to assess how well a process fits within specified limits or tolerances. It evaluates process capability based on the distribution of the data and is separate from the immediate purpose of the X-bar and R charts, which focus more on real-time monitoring and control of the process.

In contrast, elements such as the sample mean, range, and subgroup size are integral to the X-bar and R control charts. The sample mean indicates the central tendency of the process, the range provides insight into the variability within each subgroup, and the subgroup size is essential for calculating both the mean and the range accurately. Each of these components plays a crucial role in understanding and maintaining process stability.