| Year | Value | | 2021 | 1.531 | | 2020 | 1.55 | | 2019 | 1.568 | | 2018 | 1.587 | | 2017 | 1.61 | | 2016 | 1.633 | | 2015 | 1.643 | | 2014 | 1.665 | | 2013 | 1.682 | | 2012 | 1.713 | | 2011 | 1.74 | | 2010 | 1.762 | | 2009 | 1.8 | | 2008 | 1.833 | | 2007 | 1.871 | | 2006 | 1.902 | | 2005 | 1.931 | | 2004 | 1.964 | | 2003 | 2.004 | | 2002 | 2.038 | | 2001 | 2.086 | | 2000 | 2.196 | | 1999 | 2.327 | | 1998 | 2.376 | | 1997 | 2.393 | | 1996 | 2.325 | | 1995 | 2.36 | | 1994 | 2.35 | | 1993 | 2.402 | | 1992 | 2.448 | | 1991 | 2.622 | | 1990 | 2.793 | | 1989 | 2.78 | | 1988 | 2.744 | | 1987 | 2.769 | | 1986 | 2.862 | | 1985 | 3.004 | | 1984 | 3.055 | | 1983 | 3.248 | | 1982 | 3.451 | | 1981 | 3.392 | | 1980 | 3.345 | | 1979 | 3.331 | | 1978 | 3.419 | | 1977 | 3.646 | | 1976 | 3.773 | | 1975 | 3.851 | | 1974 | 3.98 | | 1973 | 4.14 | | 1972 | 4.355 | | 1971 | 4.775 | | 1970 | 5.007 | | 1969 | 5.263 | | 1968 | 5.498 | | 1967 | 5.865 | | 1966 | 6.273 | | 1965 | 6.535 | | 1964 | 6.661 | | 1963 | 6.735 | | 1962 | 6.687 | | 1961 | 6.924 | | 1960 | 7.169 |
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