| Year | Value | | 2021 | 2.427 | | 2020 | 2.465 | | 2019 | 2.504 | | 2018 | 2.546 | | 2017 | 2.584 | | 2016 | 2.59 | | 2015 | 2.637 | | 2014 | 2.686 | | 2013 | 2.737 | | 2012 | 2.78 | | 2011 | 2.814 | | 2010 | 2.853 | | 2009 | 2.952 | | 2008 | 3.057 | | 2007 | 3.177 | | 2006 | 3.208 | | 2005 | 3.242 | | 2004 | 3.337 | | 2003 | 3.502 | | 2002 | 3.706 | | 2001 | 3.91 | | 2000 | 4.115 | | 1999 | 4.247 | | 1998 | 4.419 | | 1997 | 4.592 | | 1996 | 4.769 | | 1995 | 4.952 | | 1994 | 5.142 | | 1993 | 5.319 | | 1992 | 5.493 | | 1991 | 5.662 | | 1990 | 5.83 | | 1989 | 5.997 | | 1988 | 6.17 | | 1987 | 6.36 | | 1986 | 6.548 | | 1985 | 6.703 | | 1984 | 6.837 | | 1983 | 6.949 | | 1982 | 7.047 | | 1981 | 7.126 | | 1980 | 7.185 | | 1979 | 7.227 | | 1978 | 7.263 | | 1977 | 7.295 | | 1976 | 7.33 | | 1975 | 7.37 | | 1974 | 7.426 | | 1973 | 7.484 | | 1972 | 7.539 | | 1971 | 7.556 | | 1970 | 7.583 | | 1969 | 7.597 | | 1968 | 7.627 | | 1967 | 7.658 | | 1966 | 7.655 | | 1965 | 7.658 | | 1964 | 7.672 | | 1963 | 7.654 | | 1962 | 7.636 | | 1961 | 7.634 | | 1960 | 7.626 |
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