| Year | Value | | 2021 | 2.618 | | 2020 | 2.651 | | 2019 | 2.688 | | 2018 | 2.73 | | 2017 | 2.777 | | 2016 | 2.829 | | 2015 | 2.886 | | 2014 | 2.947 | | 2013 | 3.01 | | 2012 | 3.076 | | 2011 | 3.143 | | 2010 | 3.213 | | 2009 | 3.287 | | 2008 | 3.364 | | 2007 | 3.447 | | 2006 | 3.533 | | 2005 | 3.558 | | 2004 | 3.633 | | 2003 | 3.721 | | 2002 | 3.802 | | 2001 | 3.89 | | 2000 | 3.991 | | 1999 | 4.09 | | 1998 | 4.149 | | 1997 | 4.219 | | 1996 | 4.336 | | 1995 | 4.409 | | 1994 | 4.494 | | 1993 | 4.592 | | 1992 | 4.713 | | 1991 | 4.81 | | 1990 | 4.89 | | 1989 | 4.963 | | 1988 | 5.028 | | 1987 | 5.087 | | 1986 | 5.139 | | 1985 | 5.189 | | 1984 | 5.238 | | 1983 | 5.289 | | 1982 | 5.346 | | 1981 | 5.408 | | 1980 | 5.474 | | 1979 | 5.542 | | 1978 | 5.61 | | 1977 | 5.673 | | 1976 | 5.732 | | 1975 | 5.786 | | 1974 | 5.835 | | 1973 | 5.88 | | 1972 | 5.923 | | 1971 | 5.964 | | 1970 | 6.004 | | 1969 | 6.045 | | 1968 | 6.086 | | 1967 | 6.127 | | 1966 | 6.168 | | 1965 | 6.208 | | 1964 | 6.245 | | 1963 | 6.279 | | 1962 | 6.309 | | 1961 | 6.335 | | 1960 | 6.358 |
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