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.

SPC Training Video

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Do you want to quickly improve your SPC methods…?
Do you want to apply SPC correctly to your processes…?

If you answered yes to any of the above questions then you need to take this class.

Download this SPC training video and use it to train all of your employees who need SPC training. These employees include quality department employees, process and design engineers, operators, technicians, or any one who works on or with your processes.

Course Description

Statistical Process Control (SPC) is a scientific visual method used to monitor, control and improve processes by eliminating special cause variation from manufacturing, service and financial processes. It is a key Six Sigma tool.  
SPC directs the process champion when to make take action to adjust or investigate the process. Control chart SPC also prevents inappropriate or unnecessary process adjustments. SPC tells you when there is special cause variation or when the process is drifting. SPC can be used to catch machine wear, operator setup issues, raw material changes, and differences between similar machines. Overall, one uses SPC to prevent defects from occurring and tracks process improvement.
You will learn... 
  • What is control chart SPC
  • Why use SPC
  • Process vs. product focus SPC
  • Types of SPC charts
  • Examples of control chart SPC uses
  • SPC and normal distribution
  • Making the SPC chart and the math behind SPC
  • Setting up the SPC charts
  • Interpreting the SPC charts
  • Special Cause Variation
  • SPC chart rules
  • Dynamic vs. Static Limits
  • Spec limits and process capability
  • Sampling frequency and sample size
  • Moving Range Chart and Calculations

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For more details on the SPC course agenda, click here.

(Course created by Quality Assurance Solutions)

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

Quality Assurance Solutions
Robert Broughton
(805) 419-3344

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