XbarS chart

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An Xbar-S chart is a specific type of control chart that depicts the variability of average characteristics of a process over time when variables are collected in sub-groups. Xbar-S charts are generally employed for plotting variability of sub-groups with sizes greater than 10. Xbar-R chart is used for plotting variability when sub-group size is less than 10.

Xbar-S charts plot the process mean (the Xbar chart) and process standard deviation (the S chart) over time for variables in sub-groups. Both the Xbar and S chart must be seen together to interpret the stability of the process. The S chart must be examined first as the control limits of the Xbar chart are determined considering both the process spread and center. Process variation, which is a characteristic of spread, must be in control to correctly interpret the Xbart chart. If data-points in the S chart are outside control limits, then the limits on the Xbart chart may be inaccurate and may falsely indicate an out-of-control condition. As in other types of control charts, data-points outside of control limits in a Xbar-S chart indicate special causes.