| Year | Value | | 2021 | 1.641 | | 2020 | 1.649 | | 2019 | 1.703 | | 2018 | 1.749 | | 2017 | 1.736 | | 2016 | 1.709 | | 2015 | 1.777 | | 2014 | 1.767 | | 2013 | 1.748 | | 2012 | 1.767 | | 2011 | 1.8 | | 2010 | 1.81 | | 2009 | 1.825 | | 2008 | 1.842 | | 2007 | 1.881 | | 2006 | 1.927 | | 2005 | 1.974 | | 2004 | 2.002 | | 2003 | 2.023 | | 2002 | 2.103 | | 2001 | 2.183 | | 2000 | 2.255 | | 1999 | 2.331 | | 1998 | 2.409 | | 1997 | 2.465 | | 1996 | 2.516 | | 1995 | 2.579 | | 1994 | 2.617 | | 1993 | 2.672 | | 1992 | 2.719 | | 1991 | 2.815 | | 1990 | 2.905 | | 1989 | 3.01 | | 1988 | 3.112 | | 1987 | 3.234 | | 1986 | 3.342 | | 1985 | 3.471 | | 1984 | 3.6 | | 1983 | 3.723 | | 1982 | 3.838 | | 1981 | 3.943 | | 1980 | 4.037 | | 1979 | 4.12 | | 1978 | 4.195 | | 1977 | 4.267 | | 1976 | 4.339 | | 1975 | 4.415 | | 1974 | 4.502 | | 1973 | 4.646 | | 1972 | 4.746 | | 1971 | 4.857 | | 1970 | 4.972 | | 1969 | 5.079 | | 1968 | 5.202 | | 1967 | 5.368 | | 1966 | 5.529 | | 1965 | 5.675 | | 1964 | 5.818 | | 1963 | 5.929 | | 1962 | 5.995 | | 1961 | 6.044 | | 1960 | 6.061 |
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