| Year | Value | | 2021 | 8.151 | | 2020 | 8.061 | | 2019 | 8.008 | | 2018 | 7.956 | | 2017 | 7.9 | | 2016 | 7.82 | | 2015 | 7.572 | | 2014 | 7.502 | | 2013 | 7.335 | | 2012 | 7.281 | | 2011 | 7.021 | | 2010 | 7.058 | | 2009 | 6.883 | | 2008 | 6.963 | | 2007 | 7.005 | | 2006 | 7.242 | | 2005 | 7.233 | | 2004 | 7.946 | | 2003 | 7.272 | | 2002 | 7.14 | | 2001 | 7.084 | | 2000 | 7.131 | | 1999 | 6.904 | | 1998 | 6.815 | | 1997 | 6.827 | | 1996 | 6.836 | | 1995 | 6.85 | | 1994 | 6.943 | | 1993 | 6.99 | | 1992 | 7.098 | | 1991 | 7.233 | | 1990 | 7.348 | | 1989 | 7.425 | | 1988 | 7.465 | | 1987 | 7.532 | | 1986 | 7.535 | | 1985 | 7.576 | | 1984 | 7.676 | | 1983 | 9.757 | | 1982 | 8.003 | | 1981 | 8.108 | | 1980 | 8.226 | | 1979 | 8.268 | | 1978 | 8.285 | | 1977 | 8.28 | | 1976 | 7.782 | | 1975 | 7.756 | | 1974 | 7.827 | | 1973 | 7.879 | | 1972 | 7.941 | | 1971 | 7.982 | | 1970 | 7.997 | | 1969 | 8.145 | | 1968 | 8.315 | | 1967 | 8.757 | | 1966 | 8.78 | | 1965 | 8.772 | | 1964 | 8.788 | | 1963 | 9.268 | | 1962 | 10.006 | | 1961 | 10.738 | | 1960 | 11.431 |
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