| Year | Value | | 2020 | 0.26689750335179 | | 2019 | 0.2725416966084 | | 2018 | 0.28662976279223 | | 2017 | 0.28329942790041 | | 2016 | 0.23239317779437 | | 2015 | 0.23481448692192 | | 2014 | 0.23019029795394 | | 2013 | 0.23298684450261 | | 2012 | 0.23599966335709 | | 2011 | 0.23937643578366 | | 2010 | 0.18965687278576 | | 2009 | 0.19605584319427 | | 2008 | 0.22361447076325 | | 2007 | 0.23088816037261 | | 2005 | 0.24602969578428 | | 2004 | 0.19111031269469 | | 2003 | 0.23367669545607 | | 2002 | 0.24150333444674 | | 2001 | 0.25645232287839 | | 2000 | 0.26489816298567 | | 1999 | 0.27364037282564 | | 1998 | 0.28263472868421 | | 1997 | 0.29175877384578 | | 1996 | 0.30042685305752 | | 1995 | 0.30736842105263 | | 1994 | 0.34106717360618 | | 1993 | 0.3547533400027 | | 1992 | 0.32707758513946 | | 1991 | 0.33704727328756 |
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