2017 21.2115068493151
2016 21.4990963855422
2015 NA
2014 NA
2013 21.1473684210526
2012 22.0871794871795
2011 21.1864661654135
2010 21.3642361111111
2009 20.7786931818182
2008 20.997972972973
2007 21.0393518518519
2006 21.1047619047619
2005 21.5147222222222
2004 21.0467213114754
2003 21.1547945205479
2002 21.1833910034602
2001 NA