My Reading on ASQ CQA HB Part V Part 2

Charlie Chong/ Fion Zhang
Part VD1
Types of Variation
Walter Shewhart, the father of modern quality control, was concerned with the low- cost reduction of variation.
Shewhart distinguished two kinds of processes:
(1) a stable process with ―inevitable chance variation‖ and
(2) an unstable process with ―assignable cause variation.‖
Walter Andrew Shewhart.March 18, 1891 – March 11, 1967) was an
American physicist, engineer and statistician, sometimes known as the
father of statistical quality control and also related to the Shewhart cycle.
W. Edwards Deming said of him:
As a statistician, he was, like so many of the rest of us, self-taught, on a
good background of physics and mathematics.
Born in New Canton, Illinois to Anton and Esta Barney Shewhart, he
attended the University of Illinois at Urbana–Champaign before being
awarded his doctorate in physics from the University of California,
Berkeley in 1917. He married Edna Elizabeth Hart, daughter of William
Nathaniel and Isabelle "Ibie" Lippencott Hart on August 4, 1914 in Pike
County, Illinois.
• If the limits of process variation are well within the band of customer
tolerance (specification), then the product can be made and shipped
with reasonable assurance that the customer will be satisfied.
• If the limits of process variation just match the band of customer
tolerance, then the process should be monitored closely and
adjusted when necessary to maximize the amount of satisfactory
output.
• If the limits of process variation extend beyond the band of customer
tolerance, output should be inspected to determine whether it meets
customer requirements.
State of Statistical Control (Stable)
When the amount of variation can be predicted with confidence, the
process is said to be in a state of statistical control (stable). Although a
singular value cannot be predicted exactly, it can be anticipated to fall
within certain limits. Similarly, the long- term average value can be
predicted.
In an unstable process every batch of product is a source of excitement!
It is impossible to predict how much, if any, of the product will fall within
the band of customer tolerance.