| Year | Value | | 2021 | 1.399 | | 2020 | 1.411 | | 2019 | 1.424 | | 2018 | 1.436 | | 2017 | 1.448 | | 2016 | 1.461 | | 2015 | 1.473 | | 2014 | 1.485 | | 2013 | 1.496 | | 2012 | 1.508 | | 2011 | 1.522 | | 2010 | 1.538 | | 2009 | 1.555 | | 2008 | 1.574 | | 2007 | 1.598 | | 2006 | 1.63 | | 2005 | 1.675 | | 2004 | 1.74 | | 2003 | 1.826 | | 2002 | 1.933 | | 2001 | 2.06 | | 2000 | 2.202 | | 1999 | 2.35 | | 1998 | 2.499 | | 1997 | 2.641 | | 1996 | 2.772 | | 1995 | 2.892 | | 1994 | 3.001 | | 1993 | 3.104 | | 1992 | 3.203 | | 1991 | 3.302 | | 1990 | 3.399 | | 1989 | 3.495 | | 1988 | 3.589 | | 1987 | 3.683 | | 1986 | 3.781 | | 1985 | 3.888 | | 1984 | 4.014 | | 1983 | 4.161 | | 1982 | 4.328 | | 1981 | 4.512 | | 1980 | 4.703 | | 1979 | 4.891 | | 1978 | 5.064 | | 1977 | 5.216 | | 1976 | 5.347 | | 1975 | 5.462 | | 1974 | 5.571 | | 1973 | 5.686 | | 1972 | 5.813 | | 1971 | 5.953 | | 1970 | 6.101 | | 1969 | 6.249 | | 1968 | 6.387 | | 1967 | 6.508 | | 1966 | 6.607 | | 1965 | 6.688 | | 1964 | 6.758 | | 1963 | 6.821 | | 1962 | 6.88 | | 1961 | 6.931 | | 1960 | 6.967 |
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