| Year | Value | | 2021 | 1.389 | | 2020 | 1.394 | | 2019 | 1.398 | | 2018 | 1.412 | | 2017 | 1.426 | | 2016 | 1.496 | | 2015 | 1.576 | | 2014 | 1.673 | | 2013 | 1.51 | | 2012 | 1.744 | | 2011 | 1.739 | | 2010 | 1.835 | | 2009 | 2.009 | | 2008 | 1.99 | | 2007 | 1.852 | | 2006 | 1.786 | | 2005 | 2.05 | | 2004 | 1.932 | | 2003 | 1.921 | | 2002 | 1.998 | | 2001 | 2.088 | | 2000 | 2.097 | | 1999 | 2.133 | | 1998 | 2.335 | | 1997 | 2.399 | | 1996 | 2.36 | | 1995 | 2.504 | | 1994 | 2.482 | | 1993 | 2.728 | | 1992 | 2.793 | | 1991 | 2.604 | | 1990 | 2.527 | | 1989 | 2.484 | | 1988 | 2.478 | | 1987 | 2.529 | | 1986 | 2.587 | | 1985 | 2.63 | | 1984 | 2.685 | | 1983 | 2.737 | | 1982 | 2.802 | | 1981 | 2.853 | | 1980 | 2.901 | | 1979 | 2.983 | | 1978 | 3.063 | | 1977 | 3.17 | | 1976 | 3.221 | | 1975 | 3.259 | | 1974 | 3.526 | | 1973 | 3.74 | | 1972 | 3.89 | | 1971 | 3.974 | | 1970 | 4.069 | | 1969 | 4.182 | | 1968 | 4.28 | | 1967 | 4.368 | | 1966 | 4.438 | | 1965 | 4.542 | | 1964 | 4.639 | | 1963 | 4.729 | | 1962 | 4.799 | | 1961 | 4.866 | | 1960 | 4.818 |
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