RE: SPC - independent data

[Rip Stauffer <rstauffer@bluefirepartners.com>]

In my previous post I mentioned that "data should be independent" when used
in control charts. I always get in trouble if I try to give the short
answer.

Dr. Burns is correct (as is Dr. Wheeler) in stating that autocorrelated data
may be useful in some charts. It depends on the data and the chart, and the
question you are trying to answer with the analysis. For the commonly-used
"attribute" charts (c, u, p, np) to be useful, the data do need to fit the
assumptions for the underlying distributions, or the limits will be wrong. 

I think the issue in this discussion is the clustering of messages, caused
by reaction to some particular thread, which results in a flurry of
messages. Months or time periods with interesting threads tend to contain
higher numbers of messages. Months or time periods without interesting
threads tend to generate fewer responses, and lower numbers. 

Although I think this discussion has been instructive, maybe, for those
beginning to dabble in SPC, I'm not sure we haven't drifted from our
purpose. Can we learn more from this analysis? I believe the point has been
made that some threads are more interesting than others. I don't know
whether we have established anything about the demise of the DEN; I'm not
sure we should spend a great deal of energy deciding whether the DEN is
dead. Instead, let's start working some of these interesting threads, and
see whether we can't all learn something.

I, for one, would be very interested in having Myron bring us up to date on
his latest learnings from Feuerstein. I would also be interested in some
discussion (from a systems viewpoint) on the problems in the
economy--especially the decline of the labor market and the impact of
globalization. Of course, my viewpoint is biased by my living in the U.S.;
we have an opportunity in this forum to tap into some fine international
brains and maybe gain some insight that might make a difference.

Best regards to all,

Rip

Rip Stauffer, Senior Consultant
BlueFire Partners
1300 Fifth St. Towers, 150 So. Fifth St.
Minneapolis, MN 55402
612-344-1027
mailto:rstauffer@bluefirepartners.com
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