| Year | Value | | 2020 | 4.577 | | 2019 | 4.404 | | 2018 | 4.021 | | 2017 | 3.878 | | 2016 | 3.824 | | 2015 | 3.847 | | 2014 | 3.802 | | 2013 | 3.812 | | 2012 | 3.825 | | 2011 | 3.836 | | 2010 | 3.758 | | 2009 | 3.597 | | 2008 | 3.544 | | 2007 | 3.559 | | 2006 | 3.623 | | 2005 | 3.543 | | 2004 | 3.441 | | 2003 | 3.22 | | 2002 | 3.126 | | 2001 | 3.127 | | 2000 | 3.122 | | 1999 | 3.039 | | 1998 | 2.913 | | 1997 | 2.859 | | 1996 | 2.74 | | 1995 | 4.085 | | 1994 | 4.091 | | 1993 | 4.041 | | 1992 | 3.982 | | 1991 | 3.931 | | 1990 | 2.047 |
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