| Year | Value | | 2021 | 7.638 | | 2020 | 5.916 | | 2019 | 5.636 | | 2018 | 5.569 | | 2017 | 5.445 | | 2016 | 5.398 | | 2015 | 5.446 | | 2014 | 5.481 | | 2013 | 5.436 | | 2012 | 5.464 | | 2011 | 5.506 | | 2010 | 5.517 | | 2009 | 5.525 | | 2008 | 5.529 | | 2007 | 5.545 | | 2006 | 5.584 | | 2005 | 5.6 | | 2004 | 5.539 | | 2003 | 5.536 | | 2002 | 5.585 | | 2001 | 5.593 | | 2000 | 5.621 | | 1999 | 5.642 | | 1998 | 5.686 | | 1997 | 5.734 | | 1996 | 5.776 | | 1995 | 5.852 | | 1994 | 5.932 | | 1993 | 6.02 | | 1992 | 6.137 | | 1991 | 6.255 | | 1990 | 6.382 | | 1989 | 6.616 | | 1988 | 6.736 | | 1987 | 6.923 | | 1986 | 7.145 | | 1985 | 7.376 | | 1984 | 7.622 | | 1983 | 7.887 | | 1982 | 8.113 | | 1981 | 8.304 | | 1980 | 8.423 | | 1979 | 8.525 | | 1978 | 8.613 | | 1977 | 8.638 | | 1976 | 8.685 | | 1975 | 8.76 | | 1974 | 8.828 | | 1973 | 8.816 | | 1972 | 8.949 | | 1971 | 9.063 | | 1970 | 9.146 | | 1969 | 9.256 | | 1968 | 9.363 | | 1967 | 9.457 | | 1966 | 9.539 | | 1965 | 9.6 | | 1964 | 9.664 | | 1963 | 9.7 | | 1962 | 9.867 | | 1961 | 10.051 | | 1960 | 10.255 |
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