Looking at the XPath examples I realized many of my problems arise when I use a "group by" variable in the platform.
Taking the example I find in the XPath link:
 
This script works fine
 
// not grouped by sex
dt = Open(   "$SAMPLE_DATA\Big Class.jmp" );
ow = dt << Oneway(
	Y( :height ),
	X( :age ),
	Means and Std Dev( 1 ),
	Mean Error Bars( 1 ),
	Std Dev Lines( 1 ),
	Mean of Means( 1 )
);
testXML = Report (ow) << Get XML;
Write( testXML );
/* Return a list of NumberColBox references */
ncbList = Report( ow )["Means and Std Deviations"] << XPath( "//NumberColBox" );
If I group by sex however, get XML already throws an error. How to handle this?
 
// group by sex
dt = Open(   "$SAMPLE_DATA\Big Class.jmp" );
ow = dt << Oneway(
	Y( :height ),
	X( :age ),
	By( :sex ),
	Means and Std Dev( 1 ),
	Mean Error Bars( 1 ),
	Std Dev Lines( 1 ),
	Mean of Means( 1 )
);
// get XML fails for this platform
testXML = Report (ow) << Get XML;
Write( testXML );
/* Return a list of NumberColBox references */
ncbList = Report( ow )[???]["Means and Std Deviations"] << XPath( "//NumberColBox" );