2017 NA
2016 NA
2015 NA
2014 NA
2013 NA
2012 NA
2011 22.3
2010 23.0005479452055
2009 23.2260273972603
2008 23.2568306010929
2007 23.6983050847458
2006 24.0632876712329
2005 23.7271232876712
2004 23.3494535519126
2003 23.5879452054795
2002 23.8630136986301
2001 24.0117808219178