private static String getCategory(String b1, String b2)
		throws Exception
	{
		if( b1.equals("NA") && b2.equals("NA"))
			return "NA_for_both";
		
		if( b1.equals("NA") && ! b2.equals("NA"))
			return "NA_for_first";
		
		if( ! b1.equals("NA") && b2.equals("NA"))
			return "NA_for_second";
					
		if( !b1.equals("NA") && !b2.equals("NA"))
			return "extraction_for_both";
		
		throw new Exception("Logic error");
	}