| Year | Value | | 2021 | 4.95 | | 2020 | 4.442 | | 2019 | 4.481 | | 2018 | 4.479 | | 2017 | 4.437 | | 2016 | 4.367 | | 2015 | 4.253 | | 2014 | 4.218 | | 2013 | 4.177 | | 2012 | 4.115 | | 2011 | 4.003 | | 2010 | 3.895 | | 2009 | 3.833 | | 2008 | 3.85 | | 2007 | 3.835 | | 2006 | 3.731 | | 2005 | 3.664 | | 2004 | 3.578 | | 2003 | 3.611 | | 2002 | 3.626 | | 2001 | 3.54 | | 2000 | 3.526 | | 1999 | 3.603 | | 1998 | 3.604 | | 1997 | 3.637 | | 1996 | 3.597 | | 1995 | 3.568 | | 1994 | 3.582 | | 1993 | 3.507 | | 1992 | 3.493 | | 1991 | 3.813 | | 1990 | 3.908 | | 1989 | 3.977 | | 1988 | 3.941 | | 1987 | 3.885 | | 1986 | 3.816 | | 1985 | 3.72 |
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