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#### stm08007

Hello,

I feel I am growing more and more comfortable with control charting, but I am trying to better understand why there are seemingly so many different standard deviations.

If I had 50-5 piece samples and wanted to do an X-bar and R chart, why should I use sigma hat for my standard deviation estimator? Why can't I just use the calculation for sample standard deviation? Same with the R-bar chart--- why can't I use the sample standard deviation of the range measurements-- why must I calculate sigma_R hat?

Hopefully this question makes sense-- I just want a clear explanation of why the "basic" standard deviation formulas (population and sample) we learn in high school would be inappropriate?

One other question... my book talks about "standard deviation of the sample average"--- I see others talk about "standard error of the mean"-- are these terms interchangeable?

I feel I am growing more and more comfortable with control charting, but I am trying to better understand why there are seemingly so many different standard deviations.

If I had 50-5 piece samples and wanted to do an X-bar and R chart, why should I use sigma hat for my standard deviation estimator? Why can't I just use the calculation for sample standard deviation? Same with the R-bar chart--- why can't I use the sample standard deviation of the range measurements-- why must I calculate sigma_R hat?

Hopefully this question makes sense-- I just want a clear explanation of why the "basic" standard deviation formulas (population and sample) we learn in high school would be inappropriate?

One other question... my book talks about "standard deviation of the sample average"--- I see others talk about "standard error of the mean"-- are these terms interchangeable?

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