| Year | Value | | 2021 | 1.331 | | 2020 | 1.341 | | 2019 | 1.352 | | 2018 | 1.378 | | 2017 | 1.41 | | 2016 | 1.435 | | 2015 | 1.469 | | 2014 | 1.51 | | 2013 | 1.547 | | 2012 | 1.586 | | 2011 | 1.597 | | 2010 | 1.576 | | 2009 | 1.567 | | 2008 | 1.579 | | 2007 | 1.578 | | 2006 | 1.571 | | 2005 | 1.585 | | 2004 | 1.566 | | 2003 | 1.568 | | 2002 | 1.567 | | 2001 | 1.588 | | 2000 | 1.612 | | 1999 | 1.664 | | 1998 | 1.784 | | 1997 | 1.864 | | 1996 | 1.893 | | 1995 | 1.898 | | 1994 | 1.945 | | 1993 | 1.987 | | 1992 | 2.026 | | 1991 | 2.047 | | 1990 | 2.09 | | 1989 | 2.152 | | 1988 | 2.192 | | 1987 | 2.289 | | 1986 | 2.411 | | 1985 | 2.524 | | 1984 | 2.664 | | 1983 | 2.827 | | 1982 | 2.997 | | 1981 | 3.183 | | 1980 | 3.362 | | 1979 | 3.508 | | 1978 | 3.66 | | 1977 | 3.897 | | 1976 | 4.152 | | 1975 | 4.396 | | 1974 | 4.634 | | 1973 | 4.899 | | 1972 | 5.154 | | 1971 | 5.356 | | 1970 | 5.55 | | 1969 | 5.729 | | 1968 | 5.888 | | 1967 | 6.01 | | 1966 | 6.108 | | 1965 | 6.192 | | 1964 | 6.248 | | 1963 | 6.291 | | 1962 | 6.277 | | 1961 | 6.262 | | 1960 | 6.248 |
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