| Year | Value | | 2021 | 2.395 | | 2020 | 2.484 | | 2019 | 2.61 | | 2018 | 2.743 | | 2017 | 2.857 | | 2016 | 2.973 | | 2015 | 3.029 | | 2014 | 3.088 | | 2013 | 3.151 | | 2012 | 3.219 | | 2011 | 3.292 | | 2010 | 3.375 | | 2009 | 3.465 | | 2008 | 3.571 | | 2007 | 3.716 | | 2006 | 3.845 | | 2005 | 3.974 | | 2004 | 4.134 | | 2003 | 4.25 | | 2002 | 4.371 | | 2001 | 4.502 | | 2000 | 4.583 | | 1999 | 4.687 | | 1998 | 4.8 | | 1997 | 4.906 | | 1996 | 5.008 | | 1995 | 5.105 | | 1994 | 5.191 | | 1993 | 5.254 | | 1992 | 5.314 | | 1991 | 5.377 | | 1990 | 5.48 | | 1989 | 5.559 | | 1988 | 5.621 | | 1987 | 5.639 | | 1986 | 5.663 | | 1985 | 5.729 | | 1984 | 5.855 | | 1983 | 5.976 | | 1982 | 6.057 | | 1981 | 6.121 | | 1980 | 6.171 | | 1979 | 6.216 | | 1978 | 6.261 | | 1977 | 6.319 | | 1976 | 6.368 | | 1975 | 6.41 | | 1974 | 6.458 | | 1973 | 6.487 | | 1972 | 6.511 | | 1971 | 6.546 | | 1970 | 6.6 | | 1969 | 6.655 | | 1968 | 6.706 | | 1967 | 6.754 | | 1966 | 6.799 | | 1965 | 6.823 | | 1964 | 6.847 | | 1963 | 6.869 | | 1962 | 6.89 | | 1961 | 6.933 | | 1960 | 6.955 |
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