Appendix 2: Introduction
to Statistical Process
Control Techniques
Quality Control Today
New Demands on Systems Require Action
Socratic SPC—Overview Q & A
So What Is Statistical Process Control?
Where Did This Idea Originate?
What Exactly Are Process Control Charts?
What Is the Purpose of Control Charts?
How Do They Work?
What’s This Relationship between Variation and Assignable Causes?
So How Are These Normal-Predictable Variance Levels Determined?
What about These Rules Violations for Determining If a Series of
Points within the Control Limits Is Unnatural?
In Control? Out of Control? What’s the Point?
What’s This about Capabilities?
Can Any Type of Process Data Be Judged Using Control Charts?
OK, I’m Getting the Idea These Control Charts Are Key to Quality
Improvement. What Specifically Do They Look Like, What Are
Their Key Features and How Are They Created?
OK, Sounds Good. How Do I Go about Making These Control Charts?
All Right! When Is the Best Time to Start?
This appendix is reproduced with permission of Statit Software, Inc. © 2007.
192 ◾  Appendix 2: Introduction to Statistical Process Control Techniques
Steps Involved in Using Statistical Process Control
Specific SPC Tools and Procedures
Identification and Data Gathering
Analysis of Selected Problem
Data Gathering and Initial Charting
Control Charts
Fundamental Concepts and Key Terms
Using Process Control Charts
Types of Charts Available for the Data Gathered
Variable Data Charts—Individual, Average and Range Charts
Combination Charts
Attribute Data Charts
Defects Data vs. Rejects Data
Attribute Charts—Defects and Rejects Charts
Conclusion—Time to Put It All Together …
AT&T’s Statistical Quality Control Standards
by Marilyn K. Hart, Ph.D. & Robert F. Hart, Ph.D.
Quality Control Today
In this era of strains on the resources and rising costs of manufacturing, it
becomes increasingly apparent that decisions must be made on facts, not
just opinions. Consequently, data must be gathered and analyzed. This is
where statistical process control (SPC) comes in. For over 70 years, the man-
ufacturing arena has benefited from the tools of SPC that have helped guide
the decision-making process. In particular, the control chart has helped
determine whether special-cause variation is present implying that action
needs to be taken to either eliminate that cause if it has a detrimental effect
on the process or to make it standard operating procedure if that cause has
a beneficial effect on the process. If no special-cause variation is found to
be present, SPC helps define the capability of the stable process to judge
whether it is operating at an acceptable level.
The strength of SPC is its simplicity. And with the use of Statit on the
computer to make the calculations and to plot the charts, the simplicity
becomes complete.
New Demands on Systems Require Action
Accountability with hard data, not fuzzy opinions, is being demanded.
Existing processes must be examined and new ones discovered. The good
news is that improved quality inherently lowers costs as it provides a better
product and/or service. Statistical Process Control provides accountability
and is an essential ingredient in this quality effort.
Statistical Process Control is not an abstract theoretical exercise for math-
ematicians. It is a hands-on endeavor by people who care about their work
and strive to improve themselves and their productivity every day. SPC charts
are a tool to assist in the management of this endeavor. The decisions about
what needs to be improved, the possible methods to improve it, and the
steps to take after getting results from the charts are all made by humans and
based on wisdom and experience. Everyone should be involved in this effort!

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