And just for what it's worth, unless you understand what you're
measuring, don't use Cp's or Cpk's (he said to the horrors of the
masses!!). This is because of the dependency of Cpk on the
specifications or limits. You can artifically drive the Cpk number
higher merely by widening the limits.
It also has a tendency, bu itself, to suggest a process is in control,
when in fact it is out of control. Take for example a process which has
a Cpk of 2; with a purely bimodal distribution at the plus and minus 1.0
sigma limits, one can get such a Cpk; however, I would submit to you
that the process which produced one mode is different that the process
which produced the other mode. You may get an inkling of bimodality by
including a first order statistic along with Cpk, such as range or
standard deviation. Probably the best method is to understand what the
Pearson number is (or Pearson set, as it's genrally called). This is
nothing more than a plot of skew v. kertosis and will tell you histogram
shape and a value which is akin to Cpk called the limit (either LL or
UL).
Enough of stats101...
> Bill Davis, Ph.D.
> Diamond Multimedia Systems
> Senior Scientist
> Tel. 408.325.7868
> Cell. 408.888.5650
> e-mail: [log in to unmask]
>
>
-----Original Message-----
From: [log in to unmask] [mailto:[log in to unmask]]
Sent: Wednesday, February 24, 1999 2:14 PM
To: [log in to unmask]
Subject: [TN] Cpk Question
Isn't the world of statistics wonderful. At some very large corporations
(often automotive), Cpk is
equated almost to the bottom line. A Cpk simply provides some evidence
of how well a process is
performing relative to whether it is in control (first) and whether it
is capable of producing
product meeting specified requirements (second). This requires a
processes' control limits to be
inside the product's specified acceptance limits. If you were to turn a
Cpk chart on its side and
match its control limits to to those of an X bar R chart, or process
control chart, you would get a
clear picture whether this was true.
The higher the Cpk's numeric value, the better is a process capable of
performing. Also, remember
that when a process is said to be in control, this only means it is
consistent. That is, it may be
effecting product either consistently good - or bad. When within the
Cpk's specified limits, it is
said to be in control and effecting product meeting specified
requirements. That's a very short
version of it.
Earl Moon
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