| Year | Value | | 2021 | 1.457 | | 2020 | 1.447 | | 2019 | 1.441 | | 2018 | 1.435 | | 2017 | 1.466 | | 2016 | 1.507 | | 2015 | 1.521 | | 2014 | 1.556 | | 2013 | 1.572 | | 2012 | 1.604 | | 2011 | 1.626 | | 2010 | 1.633 | | 2009 | 1.651 | | 2008 | 1.648 | | 2007 | 1.616 | | 2006 | 1.579 | | 2005 | 1.542 | | 2004 | 1.519 | | 2003 | 1.497 | | 2002 | 1.469 | | 2001 | 1.475 | | 2000 | 1.487 | | 1999 | 1.51 | | 1998 | 1.528 | | 1997 | 1.535 | | 1996 | 1.538 | | 1995 | 1.54 | | 1994 | 1.545 | | 1993 | 1.548 | | 1992 | 1.541 | | 1991 | 1.522 | | 1990 | 1.509 | | 1989 | 1.506 | | 1988 | 1.489 | | 1987 | 1.472 | | 1986 | 1.457 | | 1985 | 1.449 | | 1984 | 1.451 | | 1983 | 1.451 | | 1982 | 1.45 | | 1981 | 1.453 | | 1980 | 1.487 | | 1979 | 1.504 | | 1978 | 1.514 | | 1977 | 1.523 | | 1976 | 1.535 | | 1975 | 1.562 | | 1974 | 1.631 | | 1973 | 1.71 | | 1972 | 1.811 | | 1971 | 1.933 | | 1970 | 2.022 | | 1969 | 2.108 | | 1968 | 2.176 | | 1967 | 2.256 | | 1966 | 2.34 | | 1965 | 2.428 | | 1964 | 2.498 | | 1963 | 2.515 | | 1962 | 2.459 | | 1961 | 2.377 | | 1960 | 2.27 |
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