| Year | Value | | 2021 | 1.626 | | 2020 | 1.631 | | 2019 | 1.643 | | 2018 | 1.651 | | 2017 | 1.662 | | 2016 | 1.678 | | 2015 | 1.682 | | 2014 | 1.693 | | 2013 | 1.703 | | 2012 | 1.711 | | 2011 | 1.713 | | 2010 | 1.701 | | 2009 | 1.69 | | 2008 | 1.713 | | 2007 | 1.709 | | 2006 | 1.692 | | 2005 | 1.68 | | 2004 | 1.685 | | 2003 | 1.705 | | 2002 | 1.718 | | 2001 | 1.746 | | 2000 | 1.769 | | 1999 | 1.788 | | 1998 | 1.797 | | 1997 | 1.822 | | 1996 | 1.874 | | 1995 | 1.931 | | 1994 | 2.023 | | 1993 | 2.101 | | 1992 | 2.205 | | 1991 | 2.282 | | 1990 | 2.382 | | 1989 | 2.523 | | 1988 | 2.682 | | 1987 | 2.837 | | 1986 | 2.978 | | 1985 | 3.064 | | 1984 | 3.073 | | 1983 | 3.167 | | 1982 | 3.236 | | 1981 | 3.265 | | 1980 | 3.238 | | 1979 | 3.235 | | 1978 | 3.206 | | 1977 | 3.169 | | 1976 | 3.187 | | 1975 | 3.236 | | 1974 | 3.338 | | 1973 | 3.475 | | 1972 | 3.596 | | 1971 | 3.597 | | 1970 | 3.603 | | 1969 | 3.646 | | 1968 | 3.921 | | 1967 | 4.061 | | 1966 | 4.292 | | 1965 | 4.59 | | 1964 | 4.851 | | 1963 | 5.058 | | 1962 | 5.219 | | 1961 | 5.301 | | 1960 | 5.345 |
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