| Year | Value | | 2021 | 3.795 | | 2020 | 3.886 | | 2019 | 3.963 | | 2018 | 4.043 | | 2017 | 4.112 | | 2016 | 4.214 | | 2015 | 4.322 | | 2014 | 4.427 | | 2013 | 4.529 | | 2012 | 4.634 | | 2011 | 4.744 | | 2010 | 4.855 | | 2009 | 4.986 | | 2008 | 5.129 | | 2007 | 5.266 | | 2006 | 5.425 | | 2005 | 5.577 | | 2004 | 5.712 | | 2003 | 5.819 | | 2002 | 5.937 | | 2001 | 6.114 | | 2000 | 6.318 | | 1999 | 6.511 | | 1998 | 6.638 | | 1997 | 6.83 | | 1996 | 7.02 | | 1995 | 7.525 | | 1994 | 7.795 | | 1993 | 8.048 | | 1992 | 8.272 | | 1991 | 8.459 | | 1990 | 8.606 | | 1989 | 8.713 | | 1988 | 8.786 | | 1987 | 8.833 | | 1986 | 8.858 | | 1985 | 8.864 | | 1984 | 8.853 | | 1983 | 8.828 | | 1982 | 8.793 | | 1981 | 8.752 | | 1980 | 8.71 | | 1979 | 8.671 | | 1978 | 8.52 | | 1977 | 8.504 | | 1976 | 8.444 | | 1975 | 8.401 | | 1974 | 8.335 | | 1973 | 8.304 | | 1972 | 8.299 | | 1971 | 8.264 | | 1970 | 8.238 | | 1969 | 8.215 | | 1968 | 8.196 | | 1967 | 8.166 | | 1966 | 8.127 | | 1965 | 8.067 | | 1964 | 8.038 | | 1963 | 7.981 | | 1962 | 7.963 | | 1961 | 7.963 | | 1960 | 7.938 |
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