| Year | Value | | 2021 | 1.442 | | 2020 | 1.5 | | 2019 | 1.551 | | 2018 | 1.631 | | 2017 | 1.593 | | 2016 | 1.609 | | 2015 | 1.694 | | 2014 | 1.663 | | 2013 | 1.688 | | 2012 | 1.681 | | 2011 | 1.756 | | 2010 | 1.693 | | 2009 | 1.688 | | 2008 | 1.575 | | 2007 | 1.428 | | 2006 | 1.39 | | 2005 | 1.468 | | 2004 | 1.517 | | 2003 | 1.589 | | 2002 | 1.608 | | 2001 | 1.563 | | 2000 | 1.581 | | 1999 | 1.618 | | 1998 | 1.588 | | 1997 | 1.563 | | 1996 | 1.437 | | 1995 | 1.471 | | 1994 | 1.461 | | 1993 | 1.491 | | 1992 | 1.561 | | 1991 | 1.658 | | 1990 | 1.804 | | 1989 | 1.827 | | 1988 | 1.853 | | 1987 | 1.802 | | 1986 | 1.76 | | 1985 | 1.761 | | 1984 | 1.77 | | 1983 | 1.788 | | 1982 | 1.756 | | 1981 | 1.564 | | 1980 | 1.619 | | 1979 | 1.76 | | 1978 | 1.874 | | 1977 | 2.213 | | 1976 | 2.519 | | 1975 | 2.853 | | 1974 | 3.199 | | 1973 | 3.492 | | 1972 | 3.724 | | 1971 | 3.87 | | 1970 | 4.001 | | 1969 | 4.118 | | 1968 | 4.212 | | 1967 | 4.294 | | 1966 | 4.378 | | 1965 | 4.459 | | 1964 | 4.528 | | 1963 | 4.559 | | 1962 | 4.421 | | 1961 | 4.265 | | 1960 | 4.129 |
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