| Year | Value | | 2021 | 3.867 | | 2020 | 3.933 | | 2019 | 3.997 | | 2018 | 4.056 | | 2017 | 4.112 | | 2016 | 4.164 | | 2015 | 4.217 | | 2014 | 4.273 | | 2013 | 4.335 | | 2012 | 4.407 | | 2011 | 4.486 | | 2010 | 4.571 | | 2009 | 4.656 | | 2008 | 4.737 | | 2007 | 4.812 | | 2006 | 4.881 | | 2005 | 4.934 | | 2004 | 5.036 | | 2003 | 5.143 | | 2002 | 5.233 | | 2001 | 5.306 | | 2000 | 5.397 | | 1999 | 5.516 | | 1998 | 5.633 | | 1997 | 5.743 | | 1996 | 5.841 | | 1995 | 5.939 | | 1994 | 6.005 | | 1993 | 6.069 | | 1992 | 6.156 | | 1991 | 6.242 | | 1990 | 6.338 | | 1989 | 6.413 | | 1988 | 6.467 | | 1987 | 6.52 | | 1986 | 6.555 | | 1985 | 6.587 | | 1984 | 6.623 | | 1983 | 6.646 | | 1982 | 6.634 | | 1981 | 6.622 | | 1980 | 6.614 | | 1979 | 6.621 | | 1978 | 6.607 | | 1977 | 6.608 | | 1976 | 6.603 | | 1975 | 6.586 | | 1974 | 6.567 | | 1973 | 6.548 | | 1972 | 6.538 | | 1971 | 6.541 | | 1970 | 6.541 | | 1969 | 6.503 | | 1968 | 6.503 | | 1967 | 6.504 | | 1966 | 6.497 | | 1965 | 6.494 | | 1964 | 6.487 | | 1963 | 6.481 | | 1962 | 6.496 | | 1961 | 6.474 | | 1960 | 6.483 |
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