2017 NA
2016 NA
2015 NA
2014 NA
2013 NA
2012 NA
2011 NA
2010 31.5183561643836
2009 31.0202739726027
2008 30.8150273224044
2007 30.9850282485876
2006 30.9175342465753
2005 30.9441095890411
2004 30.2144808743169
2003 30.6493150684932
2002 30.4969863013699
2001 30.9175342465753