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
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