| Year | Value | | 2021 | 11.325 | | 2020 | 10.785 | | 2019 | 8.48 | | 2018 | 8.308 | | 2017 | 8.15 | | 2016 | 8.035 | | 2015 | 7.947 | | 2014 | 7.868 | | 2013 | 7.819 | | 2012 | 7.573 | | 2011 | 7.238 | | 2010 | 7.029 | | 2009 | 6.81 | | 2008 | 6.57 | | 2007 | 6.318 | | 2006 | 6.477 | | 2005 | 6.306 | | 2004 | 6.181 | | 2003 | 6.057 | | 2002 | 5.953 | | 2001 | 5.872 | | 2000 | 5.798 | | 1999 | 5.732 | | 1998 | 5.664 | | 1997 | 5.923 | | 1996 | 5.571 | | 1995 | 5.549 | | 1994 | 5.52 | | 1993 | 5.48 | | 1992 | 5.466 | | 1991 | 5.515 | | 1990 | 5.526 | | 1989 | 5.554 | | 1988 | 5.578 | | 1987 | 5.627 | | 1986 | 5.73 | | 1985 | 5.794 | | 1984 | 5.808 | | 1983 | 5.838 | | 1982 | 5.899 | | 1981 | 5.973 | | 1980 | 6.05 | | 1979 | 6.445 | | 1978 | 6.595 | | 1977 | 6.711 | | 1976 | 6.937 | | 1975 | 7.137 | | 1974 | 7.417 | | 1973 | 7.742 | | 1972 | 8.121 | | 1971 | 8.558 | | 1970 | 9.013 | | 1969 | 9.552 | | 1968 | 10.131 | | 1967 | 10.738 | | 1966 | 11.396 | | 1965 | 12.146 | | 1964 | 12.993 | | 1963 | 13.918 | | 1962 | 14.871 | | 1961 | 15.735 | | 1960 | 16.681 |
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