| Year | Value | | 2021 | 7.36 | | 2020 | 7.056 | | 2019 | 7.072 | | 2018 | 7.037 | | 2017 | 7.029 | | 2016 | 6.896 | | 2015 | 6.98 | | 2014 | 6.979 | | 2013 | 7.092 | | 2012 | 7.181 | | 2011 | 7.371 | | 2010 | 7.369 | | 2009 | 8.987 | | 2008 | 7.77 | | 2007 | 7.631 | | 2006 | 7.576 | | 2005 | 7.489 | | 2004 | 9.451 | | 2003 | 7.765 | | 2002 | 7.86 | | 2001 | 7.958 | | 2000 | 8.101 | | 1999 | 6.657 | | 1998 | 6.799 | | 1997 | 6.651 | | 1996 | 6.469 | | 1995 | 6.343 | | 1994 | 5.981 | | 1993 | 5.786 | | 1992 | 5.88 | | 1991 | 6.086 | | 1990 | 5.923 | | 1989 | 6.649 | | 1988 | 5.882 | | 1987 | 5.825 | | 1986 | 5.738 | | 1985 | 5.827 | | 1984 | 5.863 | | 1983 | 6.025 | | 1982 | 6.142 | | 1981 | 6.299 | | 1980 | 6.468 | | 1979 | 6.636 | | 1978 | 6.79 | | 1977 | 7.158 | | 1976 | 7.116 | | 1975 | 7.181 | | 1974 | 7.25 | | 1973 | 7.28 | | 1972 | 7.321 | | 1971 | 7.269 | | 1970 | 7.587 | | 1969 | 8.216 | | 1968 | 8.392 | | 1967 | 8.594 | | 1966 | 8.884 | | 1965 | 9.206 | | 1964 | 9.573 | | 1963 | 9.814 | | 1962 | 10.096 | | 1961 | 10.507 | | 1960 | 11.09 |
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