SPC Calculations

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.

• Select a sample size from 2 – 8 parts*.
• Collect at least 25 groups of samples
• Take the data over a period of a couple days
• Select random lots and parts
• Select random operators and machines.
• Plot the data on a histogram and assure the data is normally distributed.

* Do not change the sample size after you start collecting your data and plotting the points.

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Limit Calculations

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

Here is the table for the constants. The constants depend on your sample size.

After reviewing this page on SPC calculations, see here for more on the SPC definition