Page 213 - The Handbook for Quality Management a Complete Guide to Operational Excellence

200 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 201

compensating adjustment). Since the system produces a stable result, this
situation can go unnoticed indefinitely. However, as shown by Taguchi
(1986), increased variance translates to poorer quality and higher cost.
The rationale that leads to rule #3 goes something like this: “A measure-
ment was taken and it was found to be 10 units above the desired target.
This happened because the process was set 10 units too high. I want the
average to equal the target. To accomplish this I must try to get the next
unit to be 10 units too low.” This might be used, for example, in preparing
a chemical solution. While reasonable on its face, the result of this approach
is a wildly oscillating system.
A common example of rule #4 is the “train-the-trainer” method. A
master spends a short time training a group of “experts,” who then train
others, who train others, etc. An example is on-the-job training. Another is
creating a setup by using a piece from the last job. Yet another is a gage
calibration system where standards are used to create other standards,
which are used to create still others, and so on. Just how far the final result
will be from the ideal depends on how many levels deep the scheme has
progressed.

Process Capability Studies
Process capability analysis provides an indication of whether a controlled
process is capable of reliably meeting the customer requirements. A capa-
bility analysis is a prediction, so it can only be obtained after it is verified
the process is in statistical control. Process capability analysis is a two-
stage process that involves:
1. Bringing a process into a state of statistical control for a reasonable
period of time
2. Comparing the long-term process performance to management or
engineering requirements

Process capability analysis can be done with either attribute data or
continuous data if, and only if, the process is in statistical control, and has
been for a reasonable period of time. Application of process capability
methods to processes that are not in statistical control results in unreliable
estimates of process capability and should never be done.

How to Perform a Process Capability Study
This section presents a step-by-step approach to process capability analysis
(Pyzdek, 1985). The approach makes frequent reference to materials pre-
sented elsewhere in this book.

1. Select a candidate for the study. This step should be institutionalized.
A goal of any organization should be ongoing process improvement.

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