| Year | Value | | 2021 | 7.311 | | 2020 | 5.566 | | 2019 | 5.609 | | 2018 | 5.631 | | 2017 | 5.665 | | 2016 | 5.695 | | 2015 | 5.736 | | 2014 | 5.704 | | 2013 | 5.756 | | 2012 | 5.681 | | 2011 | 5.668 | | 2010 | 5.636 | | 2009 | 5.602 | | 2008 | 5.559 | | 2007 | 5.529 | | 2006 | 5.548 | | 2005 | 5.513 | | 2004 | 5.556 | | 2003 | 5.576 | | 2002 | 5.621 | | 2001 | 5.689 | | 2000 | 5.729 | | 1999 | 5.684 | | 1998 | 5.686 | | 1997 | 5.711 | | 1996 | 5.779 | | 1995 | 5.903 | | 1994 | 5.982 | | 1993 | 6.126 | | 1992 | 6.305 | | 1991 | 6.627 | | 1990 | 6.781 | | 1989 | 6.955 | | 1988 | 7.202 | | 1987 | 7.432 | | 1986 | 7.675 | | 1985 | 7.796 | | 1984 | 7.992 | | 1983 | 8.058 | | 1982 | 8.151 | | 1981 | 8.253 | | 1980 | 8.279 | | 1979 | 8.314 | | 1978 | 8.358 | | 1977 | 8.35 | | 1976 | 8.576 | | 1975 | 8.554 | | 1974 | 8.555 | | 1973 | 8.583 | | 1972 | 8.546 | | 1971 | 8.531 | | 1970 | 8.701 | | 1969 | 8.604 | | 1968 | 8.652 | | 1967 | 8.746 | | 1966 | 8.885 | | 1965 | 9.071 | | 1964 | 9.304 | | 1963 | 9.461 | | 1962 | 9.787 | | 1961 | 10.105 | | 1960 | 10.47 |
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