2017 30.6901369863014
2016 30.9707650273224
2015 31.2558904109589
2014 31.4213698630137
2013 30.6183561643836
2012 31.0866120218579
2011 30.2169863013699
2010 30.9016438356164
2009 31.1638356164384
2008 30.8661202185792
2007 31.4328767123288
2006 31.3928571428571
2005 31.721095890411
2004 31.4525423728814
2003 NA
2002 NA
2001 NA