Page 173 - 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

Statistical Control Charts

In his landmark 1931 book, Economic Control of Quality of Manufacturing,
Shewhart described the following experiment:
Write the letter “a” on a piece of paper. Now make another a just like the
first one, then another and another until you have a series of a’s (a, a, a, ...).
You try to make all the a’s alike but you don’t; you can’t. You are willing to
accept this as an empirically established fact. But what of it? Let us see just
what this means in respect to control. Why can we not do a simple thing like
making all the a’s exactly alike? Your answer leads to a generalization which
all of us are perhaps willing to accept. It is that there are many causes of
variability among the a’s: the paper was not smooth, the lead in the pencil
was not uniform, and the unavoidable variability in your external sur-
roundings reacted upon you to introduce variation in the a’s. But are these
the only causes of variability in the a’s? Probably not.
We accept our human limitations and say that likely there are many other
factors. If we could but name all the reasons why we cannot make the a’s
alike, we would most assuredly have a better understanding of a certain part
of nature than we now have. Of course, this conception of what it means to
be able to do what we want to do is not new; it does not belong exclusively
to any one field of human thought; it is com monly accepted.
The point to be made in this simple illustration is that we are limit ed in
doing what we want to do; that to do what we set out to do, even in so simple
a thing as making a’s that are alike, requires almost infinite knowledge com-
pared with that which we now possess. It follows, there fore, since we are
thus willing to accept as axiomatic that we cannot do what we want to do
and cannot hope to understand why we cannot, that we must also accept as
axiomatic that a controlled quality will not be a constant quality. Instead, a
controlled quality must be a variable quality. This is the first characteristic.
But let us go back to the results of the experiment on the a’s and we shall
find out something more about control. Your a’s are different from my a’s;
there is something about your a’s that makes them yours and something
about my a’s that makes them mine. True, not all of your a’s are alike. Neither
are all of my a’s alike. Each group of a’s varies within a certain range and yet
each group is distinguishable from the others. This distinguishable and, as it
were, constant variability within limits is the second characteristic of control.

Shewhart goes on to define control:
A phenomenon will be said to be controlled when, through the use of past
experience, we can predict, at least within limits, how the phenomenon may
be expected to vary in the future. Here it is understood that prediction within
limits means that we can state, at least approxi mately, the probability that the
observed phenomenon will fall within the given limits.

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