Prior to conducting SPC calculations for the X bar and R chart make sure you gather your data. When gathering the data keep in mind these guidelines.
* Do not change the sample size after you start collecting your data and plotting the points.
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1) For each group of samples calculate the mean and the range.
2) Calculate the overall mean which is know as
3) Calculate the mean of all of the ranges. This is known as
4) Now you are ready to calculate the limits. See below for the equations
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Here is the table for the constants. The constants depend on your sample size.
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