## 2019

INBOX

### Calculated error

In
Expert Answers
(April 2011),
Jeffrey Vaks writes a great column—with one
minor exception. His example uses a mean of 95 and a standard deviation of 1
against a specification limit of 100-90, resulting in a C_{p}
of 1.67 and a C_{pk} of .83.

He then proceeds to show that if the
specification limits are expanded to 103.5-86.5, the
new C_{pk} would equal 2.0, which is larger
than the original C_{p} of 1.67. The only
problem is that the new specification limits call for a new calculation of C_{p}:

which gives a result
of 2.83 and thus is still larger than his calculated C_{pk}
of 2.

The simple fact is that C_{p}
is the limit of C_{pk}—always was,
always will be. He also goes on to say there is a similar effect when the
standard deviation is reduced. That also requires recalculating the C_{p}, as well as the C_{pk}.

**John
C. Finley**

*Carmel,
IN*

**Author response:** The reader’s comment is correct that with changes
in specifications and process variation, C_{p}
and C_{pk} both need to be recalculated. The
question was how C_{pk} can be larger than C_{p}.
This is what I explained: It only can be larger than the original C_{p} after the specification is expanded or the
process variation is reduced.

Indeed, I neglected to mention that the new
process has a new C_{p} that cannot be
exceeded by the new C_{pk}, though the new C_{pk} exceeds the original C_{p}.

**Jeffrey
E. Vaks**

*Roche Molecular Diagnostics
Pleasanton,
CA*

### Truth of the matter

Dean L. Gano has written a great article on a very deep topic ("Are You a Good Problem Solver?" May 2011) . But I find the following statement a bit controversial:

"People’s understanding of reality is as unique as everyone’s fingerprints, formed from every experience of their lives by a nervous system with limited and varied senses, and a brain that provides for an infinite set of perceptions using an endless set of strategies to establish their own truth."

It is OK up until the last four words—"establish their own truth." I think what the author is trying to convey is that we all have varying perceptions or interpretations of "absolute truth," depending on our own unique background of experiences among other things. This seems to suggest that truth is a moving target. This is not the case.

It’s a subtle but very important distinction that should not be lost or misconstrued. If there were no absolute truth, how can there be any science or fact? Everything would be relative and subject to infinite variance, or at least as much variance as there are humans on Earth.

**Jay
Edwardson**

*San Clemente, CA*

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