| Year | Value | | 2021 | 1.352 | | 2020 | 1.358 | | 2019 | 1.37 | | 2018 | 1.388 | | 2017 | 1.44 | | 2016 | 1.516 | | 2015 | 1.588 | | 2014 | 1.569 | | 2013 | 1.636 | | 2012 | 1.696 | | 2011 | 1.728 | | 2010 | 1.742 | | 2009 | 1.864 | | 2008 | 1.89 | | 2007 | 1.908 | | 2006 | 2.069 | | 2005 | 2.063 | | 2004 | 2.127 | | 2003 | 2.13 | | 2002 | 2.149 | | 2001 | 2.173 | | 2000 | 2.21 | | 1999 | 2.447 | | 1998 | 2.594 | | 1997 | 2.659 | | 1996 | 2.674 | | 1995 | 2.881 | | 1994 | 2.939 | | 1993 | 2.962 | | 1992 | 2.952 | | 1991 | 2.906 | | 1990 | 2.854 | | 1989 | 3.003 | | 1988 | 3.069 | | 1987 | 3.142 | | 1986 | 3.223 | | 1985 | 3.308 | | 1984 | 3.394 | | 1983 | 3.477 | | 1982 | 3.559 | | 1981 | 3.643 | | 1980 | 3.733 | | 1979 | 3.839 | | 1978 | 3.965 | | 1977 | 4.115 | | 1976 | 4.218 | | 1975 | 4.499 | | 1974 | 4.756 | | 1973 | 5.029 | | 1972 | 5.292 | | 1971 | 5.5 | | 1970 | 5.638 | | 1969 | 5.748 | | 1968 | 5.817 | | 1967 | 5.847 | | 1966 | 5.83 | | 1965 | 5.747 | | 1964 | 5.684 | | 1963 | 5.509 | | 1962 | 5.378 | | 1961 | 5.446 | | 1960 | 5.58 |
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