| Year | Value | | 2021 | 6.82 | | 2020 | 6.892 | | 2019 | 6.961 | | 2018 | 7.023 | | 2017 | 7.084 | | 2016 | 7.141 | | 2015 | 7.211 | | 2014 | 7.279 | | 2013 | 7.344 | | 2012 | 7.4 | | 2011 | 7.449 | | 2010 | 7.485 | | 2009 | 7.513 | | 2008 | 7.539 | | 2007 | 7.559 | | 2006 | 7.579 | | 2005 | 7.615 | | 2004 | 7.634 | | 2003 | 7.654 | | 2002 | 7.671 | | 2001 | 7.695 | | 2000 | 7.732 | | 1999 | 7.752 | | 1998 | 7.762 | | 1997 | 7.759 | | 1996 | 7.762 | | 1995 | 7.776 | | 1994 | 7.786 | | 1993 | 7.79 | | 1992 | 7.795 | | 1991 | 7.798 | | 1990 | 7.806 | | 1989 | 7.818 | | 1988 | 7.841 | | 1987 | 7.871 | | 1986 | 7.904 | | 1985 | 7.932 | | 1984 | 7.921 | | 1983 | 7.902 | | 1982 | 7.862 | | 1981 | 7.822 | | 1980 | 7.782 | | 1979 | 7.722 | | 1978 | 7.662 | | 1977 | 7.595 | | 1976 | 7.563 | | 1975 | 7.539 | | 1974 | 7.539 | | 1973 | 7.529 | | 1972 | 7.523 | | 1971 | 7.518 | | 1970 | 7.514 | | 1969 | 7.506 | | 1968 | 7.497 | | 1967 | 7.49 | | 1966 | 7.481 | | 1965 | 7.466 | | 1964 | 7.48 | | 1963 | 7.489 | | 1962 | 7.501 | | 1961 | 7.512 | | 1960 | 7.53 |
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