There was an error in the formula for the X-roll studies spreadsheet and 
PDF  It was pointed out by Randy Pratt.  Thanks!  The correct papers 
were not used when I started putting the web site
   http://www.myeclectic.info/SeismoPivots/seismopivots.htm
together, but now they're updated.  The new files are
X-Roll study 2.xls
X-Roll study 2.pdf
Regards,
Charles R Patton
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