2017 NA
2016 NA
2015 NA
2014 NA
2013 27.1946619217082
2012 27.3671686746988
2011 27.2356164383562
2010 28.0254335260116
2009 27.5054441260745
2008 28.0641791044776
2007 NA
2006 NA
2005 NA
2004 NA
2003 NA
2002 NA
2001 NA