| Year | Value | | 2023 | 2.609 | | 2022 | 2.653 | | 2021 | 2.699 | | 2020 | 2.745 | | 2019 | 2.798 | | 2018 | 2.858 | | 2017 | 2.916 | | 2016 | 2.969 | | 2015 | 3.041 | | 2014 | 3.116 | | 2013 | 3.187 | | 2012 | 3.255 | | 2011 | 3.341 | | 2010 | 3.431 | | 2009 | 3.521 | | 2008 | 3.621 | | 2007 | 3.756 | | 2006 | 3.899 | | 2005 | 4.035 | | 2004 | 4.162 | | 2003 | 4.296 | | 2002 | 4.41 | | 2001 | 4.518 | | 2000 | 4.601 | | 1999 | 4.687 | | 1998 | 4.795 | | 1997 | 4.966 | | 1996 | 5.149 | | 1995 | 5.331 | | 1994 | 5.507 | | 1993 | 5.638 | | 1992 | 5.751 | | 1991 | 5.856 | | 1990 | 5.931 | | 1989 | 5.973 | | 1988 | 6.03 | | 1987 | 6.086 | | 1986 | 6.139 | | 1985 | 6.192 | | 1984 | 6.247 | | 1983 | 6.298 | | 1982 | 6.342 | | 1981 | 6.397 | | 1980 | 6.46 | | 1979 | 6.523 | | 1978 | 6.586 | | 1977 | 6.64 | | 1976 | 6.691 | | 1975 | 6.737 |
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