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What does it do?  | An Xbar-S chart, or mean and standard deviation chart, is a graphical tool that allows you to view the variation in your process over time. An Xbar-S chart lets you perform statistical tests that signal when a process may be going out of control. A process that is out of control has been affected by special causes as well as common causes. The chart can also show you where to look for sources of special cause variation. The X portion of the chart contains the mean of the subgroups distributed over time. The S portion of the chart represents the standard deviation of data points in a subgroup |
Why Use? 
| The Xbar-S chart is a tool to help you determine if your process is in control by seeing if special causes are present. The presence of special cause variation indicates that factors are influencing the output of your process. Eliminating the influence of these factors will improve the performance of your process and bring it into control |
When Use? 
| An Xbar-S chart can be used in many phases of the DMAIC process when you have continuous data. Consider using an Xbar-S chart……in the Measure phase to separate common causes of variation from special causes, in the Analyze and Improve phases to ensure process stability before completing a hypothesis test, or in the Control phase to verify that the process remains in control after the sources of special cause variation have been removed. NOTE - Use Xbar-R if the sample size is small |
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