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

In the context of X-bar and R control charts, the primary focus is on the monitoring of process stability and variation. The X-bar chart tracks the average values of samples taken over time, while the R chart monitors the range of variation within those samples.

Among the options provided, the capability ratio is not a component typically found on an X-bar and R control chart. The capability ratio, often referred to as process capability index (Cp or Cpk), is used to assess how well a process meets specified limits and is more commonly associated with process capability analysis rather than control charts.

In contrast, the sample size is integral to constructing these charts, as it influences the calculation of control limits and the interpretation of the charts. The process mean is essential because the X-bar chart specifically focuses on the average of samples. Control limits are critical components, defining the thresholds within which the process variation is considered acceptable.

Thus, the capability ratio stands apart from the other four elements as a measure used for evaluating process performance rather than a component of the control charts themselves.