| Year | Value | | 2021 | 3.504 | | 2020 | 3.57 | | 2019 | 3.642 | | 2018 | 3.712 | | 2017 | 3.807 | | 2016 | 3.935 | | 2015 | 4.046 | | 2014 | 4.04 | | 2013 | 4.076 | | 2012 | 4.16 | | 2011 | 4.274 | | 2010 | 4.383 | | 2009 | 4.412 | | 2008 | 4.497 | | 2007 | 4.614 | | 2006 | 4.721 | | 2005 | 4.844 | | 2004 | 4.942 | | 2003 | 5.019 | | 2002 | 5.186 | | 2001 | 5.315 | | 2000 | 5.443 | | 1999 | 5.575 | | 1998 | 5.695 | | 1997 | 5.821 | | 1996 | 5.997 | | 1995 | 6.175 | | 1994 | 6.338 | | 1993 | 6.496 | | 1992 | 6.633 | | 1991 | 6.724 | | 1990 | 6.778 |
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