| Year | Value | | 2021 | 6.353 | | 2020 | 5.244 | | 2019 | 4.727 | | 2018 | 4.74 | | 2017 | 4.712 | | 2016 | 4.695 | | 2015 | 4.7 | | 2014 | 4.584 | | 2013 | 4.388 | | 2012 | 4.457 | | 2011 | 4.591 | | 2010 | 4.66 | | 2009 | 4.752 | | 2008 | 4.935 | | 2007 | 5.067 | | 2006 | 5.245 | | 2005 | 5.51 | | 2004 | 5.552 | | 2003 | 5.678 | | 2002 | 5.755 | | 2001 | 5.936 | | 2000 | 5.858 | | 1999 | 5.781 | | 1998 | 5.863 | | 1997 | 5.631 | | 1996 | 5.42 | | 1995 | 5.448 | | 1994 | 5.362 | | 1993 | 5.599 | | 1992 | 5.718 | | 1991 | 5.673 | | 1990 | 5.555 | | 1989 | 5.59 | | 1988 | 5.594 | | 1987 | 5.821 | | 1986 | 6.04 | | 1985 | 6.177 | | 1984 | 6.531 | | 1983 | 6.796 | | 1982 | 6.937 | | 1981 | 6.926 | | 1980 | 7.092 | | 1979 | 7.259 | | 1978 | 7.48 | | 1977 | 7.629 | | 1976 | 7.731 | | 1975 | 7.861 | | 1974 | 8.222 | | 1973 | 8.385 | | 1972 | 8.462 | | 1971 | 8.575 | | 1970 | 8.92 | | 1969 | 9.375 | | 1968 | 9.757 | | 1967 | 10.186 | | 1966 | 10.543 | | 1965 | 10.801 | | 1964 | 11.168 | | 1963 | 11.478 | | 1962 | 11.823 | | 1961 | 15.079 | | 1960 | 12.329 |
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