2017 NA
2016 NA
2015 NA
2014 NA
2013 NA
2012 NA
2011 NA
2010 NA
2009 NA
2008 22.2649253731343
2007 21.6741007194245
2006 21.9864353312303
2005 21.8991780821918
2004 21.6622807017544
2003 22.3353260869565
2002 21.5552995391705
2001 21.7630136986301