| Year | Value | | 2021 | 1.564 | | 2020 | 1.569 | | 2019 | 1.614 | | 2018 | 1.621 | | 2017 | 1.677 | | 2016 | 1.691 | | 2015 | 1.711 | | 2014 | 1.737 | | 2013 | 1.853 | | 2012 | 1.885 | | 2011 | 1.956 | | 2010 | 2.17 | | 2009 | 2.19 | | 2008 | 2.066 | | 2007 | 1.939 | | 2006 | 1.894 | | 2005 | 1.851 | | 2004 | 1.762 | | 2003 | 1.767 | | 2002 | 1.853 | | 2001 | 1.778 | | 2000 | 1.694 | | 1999 | 1.798 | | 1998 | 1.9 | | 1997 | 1.812 | | 1996 | 1.717 | | 1995 | 1.822 | | 1994 | 1.9 | | 1993 | 1.823 | | 1992 | 1.826 | | 1991 | 1.925 | | 1990 | 1.924 | | 1989 | 1.909 | | 1988 | 1.895 | | 1987 | 1.889 | | 1986 | 1.878 | | 1985 | 1.871 | | 1984 | 1.87 | | 1983 | 1.878 | | 1982 | 1.887 | | 1981 | 1.905 | | 1980 | 1.91 | | 1979 | 1.929 | | 1978 | 1.939 | | 1977 | 1.963 | | 1976 | 2.005 | | 1975 | 2.054 | | 1974 | 2.324 | | 1973 | 2.62 | | 1972 | 2.908 | | 1971 | 2.95 | | 1970 | 2.979 | | 1969 | 2.379 | | 1968 | 2.433 | | 1967 | 2.497 | | 1966 | 2.588 | | 1965 | 2.663 | | 1964 | 2.693 | | 1963 | 2.69 | | 1962 | 2.661 | | 1961 | 2.772 | | 1960 | 2.875 |
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