| Year | Value | | 2021 | 2.211 | | 2020 | 2.23 | | 2019 | 2.25 | | 2018 | 2.272 | | 2017 | 2.294 | | 2016 | 2.317 | | 2015 | 2.34 | | 2014 | 2.365 | | 2013 | 2.39 | | 2012 | 2.417 | | 2011 | 2.444 | | 2010 | 2.471 | | 2009 | 2.5 | | 2008 | 2.53 | | 2007 | 2.562 | | 2006 | 2.595 | | 2005 | 2.629 | | 2004 | 2.664 | | 2003 | 2.701 | | 2002 | 2.739 | | 2001 | 2.779 | | 2000 | 2.822 | | 1999 | 2.867 | | 1998 | 2.915 | | 1997 | 2.967 | | 1996 | 3.022 | | 1995 | 3.082 | | 1994 | 3.147 | | 1993 | 3.218 | | 1992 | 3.292 | | 1991 | 3.37 | | 1990 | 3.448 | | 1989 | 3.524 | | 1988 | 3.597 | | 1987 | 3.665 | | 1986 | 3.73 | | 1985 | 3.794 | | 1984 | 3.861 | | 1983 | 3.933 | | 1982 | 4.014 | | 1981 | 4.104 | | 1980 | 4.2 | | 1979 | 4.298 | | 1978 | 4.397 | | 1977 | 4.494 | | 1976 | 4.589 | | 1975 | 4.686 | | 1974 | 4.791 | | 1973 | 4.906 | | 1972 | 5.035 | | 1971 | 5.174 | | 1970 | 5.321 | | 1969 | 5.469 | | 1968 | 5.614 | | 1967 | 5.748 | | 1966 | 5.87 | | 1965 | 5.977 | | 1964 | 6.071 | | 1963 | 6.155 | | 1962 | 6.231 | | 1961 | 6.298 | | 1960 | 6.358 |
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