| Year | Value | | 2021 | 4.398 | | 2020 | 4.455 | | 2019 | 4.521 | | 2018 | 4.607 | | 2017 | 4.664 | | 2016 | 4.741 | | 2015 | 4.813 | | 2014 | 4.886 | | 2013 | 4.942 | | 2012 | 4.986 | | 2011 | 5.028 | | 2010 | 5.043 | | 2009 | 5.084 | | 2008 | 5.153 | | 2007 | 5.153 | | 2006 | 5.186 | | 2005 | 5.19 | | 2004 | 5.192 | | 2003 | 5.229 | | 2002 | 5.287 | | 2001 | 5.38 | | 2000 | 5.459 | | 1999 | 5.538 | | 1998 | 5.574 | | 1997 | 5.66 | | 1996 | 5.713 | | 1995 | 5.851 | | 1994 | 5.924 | | 1993 | 6.044 | | 1992 | 6.044 | | 1991 | 6.044 | | 1990 | 6.056 | | 1989 | 6.076 | | 1988 | 6.106 | | 1987 | 6.164 | | 1986 | 6.247 | | 1985 | 6.338 | | 1984 | 6.409 | | 1983 | 6.479 | | 1982 | 6.529 | | 1981 | 6.576 | | 1980 | 6.615 | | 1979 | 6.648 | | 1978 | 6.665 | | 1977 | 6.676 | | 1976 | 6.673 | | 1975 | 6.682 | | 1974 | 6.7 | | 1973 | 6.695 | | 1972 | 6.699 | | 1971 | 6.687 | | 1970 | 6.691 | | 1969 | 6.689 | | 1968 | 6.676 | | 1967 | 6.673 | | 1966 | 6.642 | | 1965 | 6.589 | | 1964 | 6.527 | | 1963 | 6.484 | | 1962 | 6.434 | | 1961 | 6.401 | | 1960 | 6.354 |
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