Page 174 - The Handbook for Quality Management a Complete Guide to Operational Excellence

160 P r o c e s s C o n t r o l Q u a n t i f y i n g P r o c e s s Va r i a t i o n 161

The critical point in this definition is that control is not defined as the
complete absence of variation. Control is simply a state where all varia-
tion is predictable variation. In all forms of prediction there is an element
of chance. Any unknown cause of variation is called a chance cause. If the
influence of any particular chance cause is very small, and if the number
of chance causes of variation is very large and relatively constant, we have
a situation where the variation is predictable within limits. You can see
from our definition above that a system such as this qualifies as a con-
trolled system. Deming uses the term “common cause” rather “chance
cause,” and we will use Deming’s term “common cause” as it is most prev-
alent in use.
An example of such a controlled system might be the production and
distribution of peaches. If you went into an orchard to a particular peach
tree at the right time of the year, you would find a tree laden with peaches
(with any luck at all). The weights of the peaches will vary. However, if you
weighed every single peach on the tree you would probably notice that
there was a distinct pattern to the weights. In fact, if you drew a small ran-
dom sample of, say, 25 peaches, you could probably predict the weights of
those peaches remaining on the tree. This predictability is the essence of a
controlled phe nomenon. The number of common causes that account
for the variation in peach weights is astronomical, but relatively constant.
A constant system of common causes results in a controlled phenomenon.
Deming demonstrated the principles behind SPC with his Red Bead
experiment, which he regularly conducted during his seminars. In this
experiment, he used a bucket of beads or marbles. Most of the beads were
white; however, a small percentage (about 10 percent) of red beads were
thoroughly mixed with the white beads in the bucket. Students volun-
teered to be process workers, who would dip a sample paddle into the
bucket and produce a day’s “production” of 50 beads for the White Bead
Company. Another student would volunteer to be an inspector. The
inspector counted the number of white beads in each operator’s daily
production. The white beads represented usable output that could be sold
to White Bead Company’s customers, while the red beads were scrap.
These results were then reported to a manager, who would invariably
chastise operators for a high number of red beads. If the operator’s pro-
duction improved on the next sample, she was rewarded; if the produc-
tion of white beads went down, more chastising.
Of course, most of the observers in the audience would chuckle heart-
ily at management’s actions, given that each production lot was merely a
dip into a bucket that held a fixed percentage of red beads. That was the
beauty of the demonstration, and Deming would draw the analogy to
general business processes, where management would chastise employ-
ees for process variation that was largely out of their control. That is, most
operational employees have no involvement with the design of their pro-
cess, the qualifications of their suppliers, the specifications, and so on.

09_Pyzdek_Ch09_p151-208.indd 161 11/21/12 1:42 AM