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.

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.

I recommend using SPC software for plotting the data, conducting the calculations and storing all the data.

If you are interested in seeing a demo of the best enterprise software for conducting real time SPC, histograms, pareto charts and capability studies please fill in the below form.

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