| Year | Value | | 2021 | 2.475 | | 2020 | 2.495 | | 2019 | 2.519 | | 2018 | 2.53 | | 2017 | 2.557 | | 2016 | 2.605 | | 2015 | 2.642 | | 2014 | 2.657 | | 2013 | 2.662 | | 2012 | 2.666 | | 2011 | 2.685 | | 2010 | 2.746 | | 2009 | 2.806 | | 2008 | 2.859 | | 2007 | 2.886 | | 2006 | 2.911 | | 2005 | 2.885 | | 2004 | 2.924 | | 2003 | 2.956 | | 2002 | 2.976 | | 2001 | 2.999 | | 2000 | 3.026 | | 1999 | 3.073 | | 1998 | 3.128 | | 1997 | 3.2 | | 1996 | 3.262 | | 1995 | 3.267 | | 1994 | 3.273 | | 1993 | 3.315 | | 1992 | 3.355 | | 1991 | 3.38 | | 1990 | 3.414 | | 1989 | 3.427 | | 1988 | 3.457 | | 1987 | 3.497 | | 1986 | 3.535 | | 1985 | 3.624 | | 1984 | 3.722 | | 1983 | 3.799 | | 1982 | 3.87 | | 1981 | 3.924 | | 1980 | 3.98 | | 1979 | 4.033 | | 1978 | 4.048 | | 1977 | 4.069 | | 1976 | 4.092 | | 1975 | 4.097 | | 1974 | 4.069 | | 1973 | 4.158 | | 1972 | 4.298 | | 1971 | 4.488 | | 1970 | 4.659 | | 1969 | 4.836 | | 1968 | 5.03 | | 1967 | 5.238 | | 1966 | 5.447 | | 1965 | 5.64 | | 1964 | 5.824 | | 1963 | 5.958 | | 1962 | 6.085 | | 1961 | 6.281 | | 1960 | 6.461 |
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