PROCESS CONTROL FOR VARIABLES: $\overline{X}$ AND *S* CHARTS

Other important control charts for variables are $\overline{X}$ and *S* charts. $\overline{X}$ and *S* charts should be used when the spread of a variable is critical. If your process capability index is close to one and you are not 100% sorting for a particular variable, you should use $\overline{X}$ and *S* charts to determine the standard deviation of your processes for that particular variable more accurately. The minimum sample size for $\overline{X}$ and *S* charts should be 10, but sample size can go up to the 100’s depending on your production volume and on your automated voluminous data capturing and analysis capability. $\overline{X}$ and *S* charts are treated and analyzed similarly to $\overline{X}$ and *R* charts in Chapter 6, but instead of ranges, standard deviations ...

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