Just more information:

There is a section in INTRODUCTION TO STATISTICAL QUALITY
CONTROL, Douglas C. Montgomery, 2nd edition, John Wiley and Sons,
New York, 1991, Pg 259 example 6-11.     It is called misapplication of
Xbar and R charts.  Says they were trying to use fraction nonconforming
on xbar and r chart format (five samples of percentages) to get more
information from the data.   Montgomery goes on to state that it is
incorrect because an inspection processes produces attribute data
governed by the binomial distribution.     He goes on to plot both.  While 
in
his plots, he doesn't see the harm in using the Xbar and R, he doesn't see 
the
added benefit.    There  doesn't seem to be anything about the Individuals 
and moving range usage -
 ----------
From:  Dave Willhard[SMTP:[log in to unmask]]
Sent:  Thursday, March 27, 1997 5:42 PM
To:  [log in to unmask]
Subject:  SPC chart for % defective

Dr. Donald Wheeler in several of his "SPC Toolkit" columns in Quality Digest
has
seemed to suggest that Individual X / Moving Range control charts can be
used to control attribute data such as percentage defective, first
pass yield, etc.  Most SPC textbooks only list standard attribute
charts (P, C, etc.) when discussing that type of data.

Does anyone currently use a "variables" type chart to control attribute
data?  I would like to use an X/MR chart rather than a P chart for % 
defective
because
it's easier for operators to plot.  I'm just wondering if it's really
valid.  Dr. Wheeler gives examples of X/MR charts used for this type
of data but never really explains the concept of why it is valid to use
them rather than standard attribute charts.

Dave Hall
Quality Engineer
Compeq International
<[log in to unmask]>

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