2017 NA
2016 NA
2015 NA
2014 NA
2013 NA
2012 NA
2011 NA
2010 NA
2009 NA
2008 30.5784153005464
2007 31.2202739726027
2006 31.193698630137
2005 31.247397260274
2004 30.1562841530055
2003 29.9739726027397
2002 30.053698630137
2001 29.8728767123288