| Year | Value | | 2021 | 1.803 | | 2020 | 1.819 | | 2019 | 1.838 | | 2018 | 1.849 | | 2017 | 1.896 | | 2016 | 1.965 | | 2015 | 2.067 | | 2014 | 2.126 | | 2013 | 2.154 | | 2012 | 2.187 | | 2011 | 2.224 | | 2010 | 2.239 | | 2009 | 2.261 | | 2008 | 2.291 | | 2007 | 2.335 | | 2006 | 2.393 | | 2005 | 2.455 | | 2004 | 2.546 | | 2003 | 2.675 | | 2002 | 2.809 | | 2001 | 2.977 | | 2000 | 3.14 | | 1999 | 3.259 | | 1998 | 3.357 | | 1997 | 3.377 | | 1996 | 3.482 | | 1995 | 3.577 | | 1994 | 3.66 | | 1993 | 3.734 | | 1992 | 3.815 | | 1991 | 3.87 | | 1990 | 3.947 | | 1989 | 4.022 | | 1988 | 4.108 | | 1987 | 4.184 | | 1986 | 4.21 | | 1985 | 4.346 | | 1984 | 4.508 | | 1983 | 4.669 | | 1982 | 4.831 | | 1981 | 4.986 | | 1980 | 5.107 | | 1979 | 5.227 | | 1978 | 5.338 | | 1977 | 5.463 | | 1976 | 5.567 | | 1975 | 5.678 | | 1974 | 5.793 | | 1973 | 5.895 | | 1972 | 6.004 | | 1971 | 6.097 | | 1970 | 6.178 | | 1969 | 6.278 | | 1968 | 6.386 | | 1967 | 6.49 | | 1966 | 6.575 | | 1965 | 6.647 | | 1964 | 6.684 | | 1963 | 6.69 | | 1962 | 6.682 | | 1961 | 6.674 | | 1960 | 6.634 |
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