Lesson 5 Statistical Process Control (SPC)

Reading Assignment:

Download and read Chapter 10 in Essentials of Quality With Cases and Experiential Exercises.  Review the Discussion Questions at the end of the chapter to be sure that you understand what you have read.

Discussion -- Statistical Process Control (SPC) Defined:
SPC is concerned with quality of conformance. SPC is defined as "the application of statistical techniques to control a process." SPC can be divided into control charting and process capability study. Control charts provide a means of determining the type of variation (common cause or assignable cause) that is present in a process. Process capability study determines the ability of the "in control" process to produce product which meets specifications.
Consider that you are manager of an operation that consists of one process.  The process produces products which you sell to customers.  What would be important to you relative to that process?  One point might be the predictability of the process.  It would be nice if the process behaved the same way day in and day out.  Being "in control" means that the process is predictable.  Control charts permit us to determine the state of control of a process.

A second important point would be the ability of the predictable process to produce products which meet specification requirements.  It is quite possible for a process to be predictabally "bad"--that is, the process consistently produces products which fail to conform to the specifications.  The appropriate actions to take would be to recenter the process on the target value if necessary, and then determine the sources of the common cause variation in the system with the purpose of minimizing that variation (common cause variation can never be eliminated).  A reduction in the variation of a properly centered process will make the process more capable of producing products which meet specifications.  An alternative way to improve the capability of a process would be to expand the specification limits.  Often you will be constrained from doing this by customer requirements.

Discussion -- Variation:
Variation is the enemy of quality. It can be partitioned between "common causes" and "assignable causes." Common cause variation exists in every process--it can be reduced by process improvement activities, but not eliminated. It is the variation that is inherent in a process that is operating as designed. Assignable cause variation is unnatural variation in a process. It should be identified and addressed.  Control charts enable us to determine the type of variation that exists in a process.

A common problem in business is managers who treat all variation as if it were due to assignable causes.  This leads to the overadjustment of processes (which increases variation), criticism of employees for variation that is out of their control, and reward of employees for being the "best" when that distinction is the result of just common cause variation.  Deming's Red Bead Experiment provides graphic proof of the folly of treating all variation as if it had an assignable cause.  The Funnel Experiment shows the result of overadjustment of a process.

A Process Control Anecdote (courtesy of G. Ferguson, Quality Digest, Dec. 1999, pp. 22-23)
A quality engineer notices a pattern on the X-bar chart of a turning process.  The point representing the measurement taken around 12:30 is always above the UCL.  All the other points are within the UCL.  Realizing that production operators are usually an invaluable source of information, the quality engineer heads to the production floor.  An interview with Charlie, the lathe operator, reveals that a relief operator, Bill, takes over at 12:00 to allow Charlie to take a lunch break.  Bill says he never has to adjust the process because it is always running fine when he takes over from Charlie.  So, you have two operators making the same part on the same machine with the same set-up and materials, but their parts are different.  The engineer asks Charlie to demonstrate how he makes a part.  Bill, watching, says he does it exactly the same way.  The engineer asks Bill to demonstrate.  As he sets up the work in the chuck, Charlie says, "Woah!  What are you doing?  That's way too much pressure.  If you clamp it down like that you'll make the Teflon distort."  Bill replies, "I've been running lathes all my life.  That's the way I always do it."  "Yeah," says Charlie, "but that was with steel parts.  Teflon parts are different.  It's easy to distort.  No wonder you were getting screwy readings on the OD."  Problem solved!
Writing Assignment:
Answer Discussion Question 1 at the end of Chapter 10 of the text.
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