| Year | Value | | 2021 | 1.778 | | 2020 | 1.796 | | 2019 | 1.819 | | 2018 | 1.848 | | 2017 | 1.881 | | 2016 | 1.915 | | 2015 | 1.947 | | 2014 | 1.969 | | 2013 | 1.978 | | 2012 | 1.973 | | 2011 | 1.957 | | 2010 | 1.937 | | 2009 | 1.921 | | 2008 | 1.918 | | 2007 | 1.933 | | 2006 | 1.969 | | 2005 | 2.022 | | 2004 | 2.086 | | 2003 | 2.154 | | 2002 | 2.22 | | 2001 | 2.284 | | 2000 | 2.346 | | 1999 | 2.413 | | 1998 | 2.489 | | 1997 | 2.578 | | 1996 | 2.677 | | 1995 | 2.785 | | 1994 | 2.897 | | 1993 | 3.007 | | 1992 | 3.111 | | 1991 | 3.206 | | 1990 | 3.29 | | 1989 | 3.363 | | 1988 | 3.428 | | 1987 | 3.488 | | 1986 | 3.546 | | 1985 | 3.608 | | 1984 | 3.675 | | 1983 | 3.751 | | 1982 | 3.84 | | 1981 | 3.943 | | 1980 | 4.067 | | 1979 | 4.216 | | 1978 | 4.389 | | 1977 | 4.58 | | 1976 | 4.784 | | 1975 | 4.99 | | 1974 | 5.183 | | 1973 | 5.355 | | 1972 | 5.5 | | 1971 | 5.619 | | 1970 | 5.719 | | 1969 | 5.812 | | 1968 | 5.912 | | 1967 | 6.026 | | 1966 | 6.156 | | 1965 | 6.297 | | 1964 | 6.44 | | 1963 | 6.572 | | 1962 | 6.683 | | 1961 | 6.771 | | 1960 | 6.836 |
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