| Year | Value | | 2020 | 4.325 | | 2019 | 4.296 | | 2018 | 3.48 | | 2017 | 3.461 | | 2016 | 3.42 | | 2015 | 3.421 | | 2014 | 3.448 | | 2013 | 3.387 | | 2012 | 3.556 | | 2011 | 3.543 | | 2010 | 3.44 | | 2009 | 3.775 | | 2008 | 3.742 | | 2007 | 3.729 | | 2006 | 3.69 | | 2005 | 3.674 | | 2004 | 3.653 | | 2003 | 2.564 | | 2002 | 2.527 | | 2001 | 2.5 | | 2000 | 2.499 | | 1999 | 2.401 | | 1998 | 2.32 | | 1997 | 2.295 | | 1996 | 2.26 | | 1995 | 2.191 | | 1994 | 3.213 | | 1993 | 3.133 | | 1992 | 3.057 | | 1991 | 2.985 | | 1990 | 2.898 |
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