2017 30.8821917808219
2016 30.5956284153005
2015 31.0120547945205
2014 30.4534246575342
2013 30.6024657534247
2012 30.7939890710383
2011 30.7317808219178
2010 31.4904109589041
2009 31.0947945205479
2008 30.8726775956284
2007 31.0315068493151
2006 30.8586301369863
2005 31.0852517985612
2004 30.7691256830601
2003 30.2241379310345
2002 NA
2001 NA