| Year | Value | | 2021 | 6.034 | | 2020 | 5.311 | | 2019 | 5.253 | | 2018 | 5.175 | | 2017 | 5.132 | | 2016 | 5.109 | | 2015 | 5.096 | | 2014 | 4.981 | | 2013 | 4.945 | | 2012 | 4.909 | | 2011 | 4.928 | | 2010 | 5.009 | | 2009 | 4.96 | | 2008 | 4.817 | | 2007 | 4.705 | | 2006 | 4.603 | | 2005 | 4.516 | | 2004 | 4.488 | | 2003 | 4.43 | | 2002 | 4.43 | | 2001 | 4.435 | | 2000 | 4.584 | | 1999 | 4.722 | | 1998 | 4.718 | | 1997 | 4.627 | | 1996 | 4.563 | | 1995 | 4.494 | | 1994 | 4.529 | | 1993 | 4.607 | | 1992 | 4.674 | | 1991 | 4.656 | | 1990 | 4.656 | | 1989 | 4.677 | | 1988 | 4.717 | | 1987 | 4.756 | | 1986 | 4.79 | | 1985 | 4.799 | | 1984 | 4.895 | | 1983 | 4.986 | | 1982 | 5.06 | | 1981 | 5.154 | | 1980 | 5.331 | | 1979 | 5.465 | | 1978 | 5.603 | | 1977 | 5.746 | | 1976 | 5.914 | | 1975 | 6.105 | | 1974 | 6.281 | | 1973 | 6.496 | | 1972 | 6.684 | | 1971 | 6.88 | | 1970 | 7.078 | | 1969 | 7.281 | | 1968 | 7.493 | | 1967 | 7.707 | | 1966 | 7.97 | | 1965 | 8.257 | | 1964 | 8.794 | | 1963 | 9.329 | | 1962 | 9.923 | | 1961 | 10.621 | | 1960 | 11.344 |
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