| Year | Value | | 2021 | 3.563 | | 2020 | 3.623 | | 2019 | 3.683 | | 2018 | 3.731 | | 2017 | 3.713 | | 2016 | 3.911 | | 2015 | 4.05 | | 2014 | 4.05 | | 2013 | 4.136 | | 2012 | 4.183 | | 2011 | 4.193 | | 2010 | 4.206 | | 2009 | 4.214 | | 2008 | 4.25 | | 2007 | 4.307 | | 2006 | 4.41 | | 2005 | 4.538 | | 2004 | 4.531 | | 2003 | 4.627 | | 2002 | 4.723 | | 2001 | 4.794 | | 2000 | 4.854 | | 1999 | 4.838 | | 1998 | 4.836 | | 1997 | 4.865 | | 1996 | 4.931 | | 1995 | 5.067 | | 1994 | 5.19 | | 1993 | 5.362 | | 1992 | 5.465 | | 1991 | 5.587 | | 1990 | 5.712 | | 1989 | 5.829 | | 1988 | 5.96 | | 1987 | 6.086 | | 1986 | 6.145 | | 1985 | 6.193 | | 1984 | 6.24 | | 1983 | 6.311 | | 1982 | 6.377 | | 1981 | 6.474 | | 1980 | 6.52 | | 1979 | 6.559 | | 1978 | 6.611 | | 1977 | 6.679 | | 1976 | 6.723 | | 1975 | 6.77 | | 1974 | 6.827 | | 1973 | 6.871 | | 1972 | 6.907 | | 1971 | 6.937 | | 1970 | 6.953 | | 1969 | 6.961 | | 1968 | 6.911 | | 1967 | 6.91 | | 1966 | 6.875 | | 1965 | 6.89 | | 1964 | 6.931 | | 1963 | 6.98 | | 1962 | 6.947 | | 1961 | 6.886 | | 1960 | 6.847 |
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