Peter, I don't believe Deming ever claimed that he "knew" this as real
data. It was only his impression from working with, and hearing from,
hundreds of companies over his long life -- companies who continually
brought him problems that he helped them work through.
I also believe he changed the percentages (making them higher for
system-caused problems) as he got older -- perhaps because he didn't think
anyone would believe the higher percentage when he first started lecturing
or perhaps because he became convinced that he had underestimated.
You'll also note that even in Out of the Crisis his quote contains the
words "In my experience ..." and "add up to proportions something like
...." which are a clue that he is not pretending to precision.
--"John Gunkler" <jgunkler@sprintmail.com>
Learning-org -- Hosted by Rick Karash <rkarash@karash.com> Public Dialog on Learning Organizations -- <http://www.learning-org.com>