| Year | Value | | 2017 | 1.739 | | 2016 | 1.747 | | 2015 | 1.753 | | 2014 | 1.758 | | 2013 | 1.764 | | 2012 | 1.771 | | 2011 | 1.781 | | 2010 | 1.796 | | 2009 | 1.818 | | 2008 | 1.847 | | 2007 | 1.882 | | 2006 | 1.925 | | 2005 | 1.976 | | 2004 | 2.033 | | 2003 | 2.097 | | 2002 | 2.163 | | 2001 | 2.232 | | 2000 | 2.3 | | 1999 | 2.365 | | 1998 | 2.426 | | 1997 | 2.483 | | 1996 | 2.536 | | 1995 | 2.588 | | 1994 | 2.639 | | 1993 | 2.693 | | 1992 | 2.754 | | 1991 | 2.822 | | 1990 | 2.902 | | 1989 | 2.995 | | 1988 | 3.101 | | 1987 | 3.217 | | 1986 | 3.342 | | 1985 | 3.471 | | 1984 | 3.6 | | 1983 | 3.723 | | 1982 | 3.838 | | 1981 | 3.943 | | 1980 | 4.037 | | 1979 | 4.12 | | 1978 | 4.195 | | 1977 | 4.267 | | 1976 | 4.339 | | 1975 | 4.415 | | 1974 | 4.502 | | 1973 | 4.6 | | 1972 | 4.712 | | 1971 | 4.836 | | 1970 | 4.972 | | 1969 | 5.12 | | 1968 | 5.272 | | 1967 | 5.424 | | 1966 | 5.57 | | 1965 | 5.703 | | 1964 | 5.818 | | 1963 | 5.91 | | 1962 | 5.981 | | 1961 | 6.031 | | 1960 | 6.061 |
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