| Year | Value | | 2021 | 2.729 | | 2020 | 2.777 | | 2019 | 2.824 | | 2018 | 2.877 | | 2017 | 2.929 | | 2016 | 2.983 | | 2015 | 3.063 | | 2014 | 3.161 | | 2013 | 3.274 | | 2012 | 3.389 | | 2011 | 3.51 | | 2010 | 3.632 | | 2009 | 3.757 | | 2008 | 3.862 | | 2007 | 3.966 | | 2006 | 4.056 | | 2005 | 4.129 | | 2004 | 4.209 | | 2003 | 4.292 | | 2002 | 4.378 | | 2001 | 4.489 | | 2000 | 4.592 | | 1999 | 4.697 | | 1998 | 4.787 | | 1997 | 4.886 | | 1996 | 5.017 | | 1995 | 5.147 | | 1994 | 5.308 | | 1993 | 5.461 | | 1992 | 5.627 | | 1991 | 5.832 | | 1990 | 6.03 | | 1989 | 6.238 | | 1988 | 6.438 | | 1987 | 6.636 | | 1986 | 6.785 | | 1985 | 6.917 | | 1984 | 7.037 | | 1983 | 7.144 | | 1982 | 7.247 | | 1981 | 7.341 | | 1980 | 7.439 | | 1979 | 7.536 | | 1978 | 7.644 | | 1977 | 7.74 | | 1976 | 7.828 | | 1975 | 7.906 | | 1974 | 7.977 | | 1973 | 8.036 | | 1972 | 8.092 | | 1971 | 8.148 | | 1970 | 8.21 | | 1969 | 8.264 | | 1968 | 8.315 | | 1967 | 8.34 | | 1966 | 8.356 | | 1965 | 8.344 | | 1964 | 8.33 | | 1963 | 8.309 | | 1962 | 8.285 | | 1961 | 8.266 | | 1960 | 8.234 |
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