| Year | Value | | 2021 | 8.955 | | 2020 | 8.509 | | 2019 | 7.6 | | 2018 | 7.68 | | 2017 | 7.692 | | 2016 | 7.947 | | 2015 | 7.543 | | 2014 | 7.447 | | 2013 | 7.494 | | 2012 | 7.394 | | 2011 | 7.51 | | 2010 | 7.661 | | 2009 | 7.41 | | 2008 | 7.467 | | 2007 | 7.748 | | 2006 | 7.369 | | 2005 | 7.423 | | 2004 | 7.48 | | 2003 | 7.864 | | 2002 | 7.518 | | 2001 | 7.549 | | 2000 | 7.6 | | 1999 | 7.615 | | 1998 | 7.651 | | 1997 | 7.778 | | 1996 | 7.541 | | 1995 | 7.535 | | 1994 | 7.403 | | 1993 | 7.631 | | 1992 | 7.595 | | 1991 | 7.536 | | 1990 | 7.743 | | 1989 | 7.841 | | 1988 | 7.96 | | 1987 | 8.157 | | 1986 | 8.34 | | 1985 | 8.547 | | 1984 | 8.746 | | 1983 | 8.601 | | 1982 | 8.695 | | 1981 | 8.582 | | 1980 | 8.722 | | 1979 | 8.646 | | 1978 | 8.681 | | 1977 | 8.76 | | 1976 | 9.026 | | 1975 | 8.983 | | 1974 | 9.154 | | 1973 | 9.183 | | 1972 | 9.277 | | 1971 | 9.364 | | 1970 | 9.151 | | 1969 | 9.478 | | 1968 | 9.296 | | 1967 | 9.076 | | 1966 | 9.001 | | 1965 | 9.062 | | 1964 | 9.093 | | 1963 | 8.917 | | 1962 | 8.924 | | 1961 | 8.724 | | 1960 | 8.844 |
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