| Year | Value | | 2021 | 3.93 | | 2020 | 3.997 | | 2019 | 4.047 | | 2018 | 4.109 | | 2017 | 4.183 | | 2016 | 4.269 | | 2015 | 4.357 | | 2014 | 4.443 | | 2013 | 4.525 | | 2012 | 4.606 | | 2011 | 4.639 | | 2010 | 4.608 | | 2009 | 4.549 | | 2008 | 4.469 | | 2007 | 4.391 | | 2006 | 4.373 | | 2005 | 4.374 | | 2004 | 4.382 | | 2003 | 4.4 | | 2002 | 4.429 | | 2001 | 4.465 | | 2000 | 4.514 | | 1999 | 4.555 | | 1998 | 4.609 | | 1997 | 4.66 | | 1996 | 4.739 | | 1995 | 4.802 | | 1994 | 4.85 | | 1993 | 4.874 | | 1992 | 4.888 | | 1991 | 4.892 | | 1990 | 4.93 | | 1989 | 4.99 | | 1988 | 5.089 | | 1987 | 5.19 | | 1986 | 5.286 | | 1985 | 5.344 | | 1984 | 5.417 | | 1983 | 5.531 | | 1982 | 5.661 | | 1981 | 5.784 | | 1980 | 5.925 | | 1979 | 6.083 | | 1978 | 6.229 | | 1977 | 6.378 | | 1976 | 6.529 | | 1975 | 6.679 | | 1974 | 6.847 | | 1973 | 7.024 | | 1972 | 7.182 | | 1971 | 7.337 | | 1970 | 7.459 | | 1969 | 7.547 | | 1968 | 7.592 | | 1967 | 7.612 | | 1966 | 7.625 | | 1965 | 7.651 | | 1964 | 7.667 | | 1963 | 7.656 | | 1962 | 7.646 | | 1961 | 7.641 | | 1960 | 7.646 |
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