| Year | Value | | 2021 | 2.814 | | 2020 | 2.869 | | 2019 | 2.919 | | 2018 | 2.959 | | 2017 | 3.016 | | 2016 | 3.062 | | 2015 | 3.108 | | 2014 | 3.157 | | 2013 | 3.216 | | 2012 | 3.302 | | 2011 | 3.411 | | 2010 | 3.47 | | 2009 | 3.508 | | 2008 | 3.567 | | 2007 | 3.643 | | 2006 | 3.735 | | 2005 | 3.833 | | 2004 | 3.939 | | 2003 | 4.038 | | 2002 | 4.125 | | 2001 | 4.248 | | 2000 | 4.39 | | 1999 | 4.452 | | 1998 | 4.561 | | 1997 | 4.669 | | 1996 | 4.777 | | 1995 | 4.884 | | 1994 | 4.991 | | 1993 | 5.099 | | 1992 | 5.208 | | 1991 | 5.317 | | 1990 | 5.483 | | 1989 | 5.532 | | 1988 | 5.566 | | 1987 | 5.593 | | 1986 | 5.607 | | 1985 | 5.624 | | 1984 | 5.632 | | 1983 | 5.656 | | 1982 | 5.697 | | 1981 | 5.673 | | 1980 | 5.65 | | 1979 | 5.669 | | 1978 | 5.673 | | 1977 | 5.681 | | 1976 | 5.691 | | 1975 | 5.687 | | 1974 | 5.663 | | 1973 | 5.648 | | 1972 | 5.722 | | 1971 | 5.797 | | 1970 | 5.87 | | 1969 | 5.925 | | 1968 | 5.974 | | 1967 | 6.027 | | 1966 | 6.071 | | 1965 | 6.123 | | 1964 | 6.175 | | 1963 | 6.182 | | 1962 | 6.194 | | 1961 | 6.2 | | 1960 | 6.208 |
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