| Year | Value | | 2020 | 3.827 | | 2019 | 4.187 | | 2018 | 4.119 | | 2017 | 4.078 | | 2016 | 3.982 | | 2015 | 3.942 | | 2014 | 4.138 | | 2013 | 4.005 | | 2012 | 4.022 | | 2011 | 4.24 | | 2010 | 4.997 | | 2009 | 4.968 | | 2008 | 4.919 | | 2007 | 4.942 | | 2006 | 4.899 | | 2005 | 4.8 | | 2004 | 4.768 | | 2003 | 4.733 | | 2002 | 4.687 | | 2001 | 4.635 | | 2000 | 4.632 | | 1999 | 4.632 | | 1998 | 4.603 | | 1997 | 4.552 | | 1996 | 4.515 | | 1995 | 4.4 | | 1994 | 4.279 | | 1993 | 4.309 | | 1992 | 4.285 | | 1991 | 4.259 | | 1990 | 4.509 | | 1989 | 4.085 | | 1988 | 4.04 | | 1987 | 4.004 | | 1986 | 3.939 | | 1985 | 3.871 |
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