2017 NA
2016 NA
2015 NA
2014 21.6151376146789
2013 22.15
2012 22.0969696969697
2011 22.310101010101
2010 22.0605042016807
2009 22.141369047619
2008 21.8627760252366
2007 22.1015
2006 22.3469208211144
2005 22.714409221902
2004 22.4019292604502
2003 NA
2002 20.6588235294118
2001 NA