2017 NA
2016 NA
2015 NA
2014 20.5090614886731
2013 20.3893150684932
2012 20.4625714285714
2011 20.6
2010 20.3457142857143
2009 20.5227138643068
2008 20.205572755418
2007 20.5904545454545
2006 20.6182584269663
2005 20.8380281690141
2004 20.6550632911392
2003 NA
2002 19.2
2001 NA