| Year | Value | | 2021 | 4.911 | | 2020 | 4.788 | | 2019 | 4.661 | | 2018 | 4.176 | | 2017 | 4.747 | | 2016 | 4.345 | | 2015 | 4.257 | | 2014 | 3.862 | | 2013 | 4.068 | | 2012 | 3.847 | | 2011 | 3.72 | | 2010 | 3.718 | | 2009 | 3.652 | | 2008 | 3.602 | | 2007 | 3.569 | | 2006 | 3.542 | | 2005 | 3.527 | | 2004 | 3.525 | | 2003 | 3.546 | | 2002 | 3.57 | | 2001 | 3.603 | | 2000 | 3.63 | | 1999 | 3.547 | | 1998 | 3.354 | | 1997 | 3.184 | | 1996 | 3.038 | | 1995 | 2.911 | | 1994 | 2.803 | | 1993 | 2.727 | | 1992 | 2.667 | | 1991 | 2.448 | | 1990 | 2.748 | | 1989 | 2.785 | | 1988 | 2.785 | | 1987 | 2.988 | | 1986 | 3.068 | | 1985 | 3.402 | | 1984 | 3.479 | | 1983 | 3.742 | | 1982 | 3.881 | | 1981 | 4.145 | | 1980 | 4.323 | | 1979 | 4.46 | | 1978 | 4.579 | | 1977 | 4.689 | | 1976 | 4.773 | | 1975 | 4.886 | | 1974 | 4.936 | | 1973 | 4.976 | | 1972 | 5.034 | | 1971 | 5.088 | | 1970 | 5.026 | | 1969 | 5.343 | | 1968 | 5.62 | | 1967 | 5.635 | | 1966 | 5.866 | | 1965 | 6.33 | | 1964 | 5.914 | | 1963 | 5.972 | | 1962 | 6.423 | | 1961 | 6.755 | | 1960 | 6.714 |
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