| Year | Value | | 2023 | 3.724 | | 2022 | 3.767 | | 2021 | 3.765 | | 2020 | 3.754 | | 2019 | 3.748 | | 2018 | 3.744 | | 2017 | 3.768 | | 2016 | 3.828 | | 2015 | 3.911 | | 2014 | 4.011 | | 2013 | 4.111 | | 2012 | 4.134 | | 2011 | 4.126 | | 2010 | 4.04 | | 2009 | 3.95 | | 2008 | 3.783 | | 2007 | 3.677 | | 2006 | 3.635 | | 2005 | 3.693 | | 2004 | 3.776 | | 2003 | 3.862 | | 2002 | 3.925 | | 2001 | 3.983 | | 2000 | 4.009 | | 1999 | 4.056 | | 1998 | 4.067 | | 1997 | 4.075 | | 1996 | 4.11 | | 1995 | 4.153 | | 1994 | 4.284 | | 1993 | 4.391 | | 1992 | 4.567 | | 1991 | 4.715 | | 1990 | 4.876 | | 1989 | 5.073 | | 1988 | 5.279 | | 1987 | 5.5 | | 1986 | 5.688 | | 1985 | 5.871 | | 1984 | 6.044 | | 1983 | 6.197 | | 1982 | 6.333 | | 1981 | 6.445 | | 1980 | 6.517 | | 1979 | 6.59 | | 1978 | 6.658 | | 1977 | 6.713 | | 1976 | 6.771 |
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