| Year | Value | | 2021 | 3.868 | | 2020 | 3.475 | | 2019 | 3.148 | | 2018 | 3.15 | | 2017 | 3.155 | | 2016 | 3.169 | | 2015 | 3.169 | | 2014 | 3.361 | | 2013 | 3.592 | | 2012 | 3.596 | | 2011 | 3.597 | | 2010 | 3.604 | | 2009 | 3.611 | | 2008 | 3.621 | | 2007 | 3.625 | | 2006 | 3.635 | | 2005 | 3.645 | | 2004 | 3.643 | | 2003 | 3.655 | | 2002 | 3.67 | | 2001 | 3.689 | | 2000 | 3.724 | | 1999 | 3.769 | | 1998 | 3.819 | | 1997 | 3.875 | | 1996 | 3.936 | | 1995 | 3.993 | | 1994 | 4.045 | | 1993 | 4.102 | | 1992 | 4.169 | | 1991 | 4.25 | | 1990 | 4.345 | | 1989 | 4.471 | | 1988 | 4.609 | | 1987 | 4.772 | | 1986 | 4.923 | | 1985 | 5.07 | | 1984 | 5.239 | | 1983 | 5.434 | | 1982 | 5.646 | | 1981 | 5.888 | | 1980 | 6.148 | | 1979 | 6.387 | | 1978 | 6.644 | | 1977 | 6.937 | | 1976 | 7.243 | | 1975 | 7.58 | | 1974 | 7.92 | | 1973 | 8.274 | | 1972 | 8.656 | | 1971 | 9.127 | | 1970 | 10.087 | | 1969 | 9.991 | | 1968 | 10.517 | | 1967 | 11.065 | | 1966 | 11.885 | | 1965 | 12.266 | | 1964 | 12.887 | | 1963 | 13.608 | | 1962 | 14.282 | | 1961 | 14.996 | | 1960 | 15.744 |
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