| Year | Value | | 2021 | 8.859 | | 2020 | 8.078 | | 2019 | 7.941 | | 2018 | 7.832 | | 2017 | 7.703 | | 2016 | 7.885 | | 2015 | 8.05 | | 2014 | 7.584 | | 2013 | 7.424 | | 2012 | 7.212 | | 2011 | 7.114 | | 2010 | 7.113 | | 2009 | 7.141 | | 2008 | 7.141 | | 2007 | 7.052 | | 2006 | 6.9 | | 2005 | 6.885 | | 2004 | 6.866 | | 2003 | 6.875 | | 2002 | 6.999 | | 2001 | 6.565 | | 2000 | 6.457 | | 1999 | 6.116 | | 1998 | 6.036 | | 1997 | 6.016 | | 1996 | 5.985 | | 1995 | 5.984 | | 1994 | 6.007 | | 1993 | 6.055 | | 1992 | 6.069 | | 1991 | 6.121 | | 1990 | 6.159 | | 1989 | 6.24 | | 1988 | 6.283 | | 1987 | 6.37 | | 1986 | 6.361 | | 1985 | 6.409 | | 1984 | 6.668 | | 1983 | 6.929 | | 1982 | 7.161 | | 1981 | 7.192 | | 1980 | 7.185 | | 1979 | 7.075 | | 1978 | 7.252 | | 1977 | 7.344 | | 1976 | 7.443 | | 1975 | 7.426 | | 1974 | 7.433 | | 1973 | 7.739 | | 1972 | 7.377 | | 1971 | 7.399 | | 1970 | 7.472 | | 1969 | 7.62 | | 1968 | 7.585 | | 1967 | 7.802 | | 1966 | 7.912 | | 1965 | 8.141 | | 1964 | 8.571 | | 1963 | 9.082 | | 1962 | 9.577 | | 1961 | 10.146 | | 1960 | 10.728 |
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