| Year | Value | | 2018 | 2.427 | | 2017 | 2.451 | | 2016 | 2.475 | | 2015 | 2.499 | | 2014 | 2.522 | | 2013 | 2.544 | | 2012 | 2.567 | | 2011 | 2.59 | | 2010 | 2.615 | | 2009 | 2.643 | | 2008 | 2.675 | | 2007 | 2.711 | | 2006 | 2.753 | | 2005 | 2.8 | | 2004 | 2.853 | | 2003 | 2.911 | | 2002 | 2.972 | | 2001 | 3.037 | | 2000 | 3.102 | | 1999 | 3.166 | | 1998 | 3.228 | | 1997 | 3.288 | | 1996 | 3.345 | | 1995 | 3.403 | | 1994 | 3.461 | | 1993 | 3.524 | | 1992 | 3.591 | | 1991 | 3.665 | | 1990 | 3.744 | | 1989 | 3.829 | | 1988 | 3.917 | | 1987 | 4.007 | | 1986 | 4.1 | | 1985 | 4.195 | | 1984 | 4.293 | | 1983 | 4.395 | | 1982 | 4.501 | | 1981 | 4.612 | | 1980 | 4.73 | | 1979 | 4.855 | | 1978 | 4.989 | | 1977 | 5.13 | | 1976 | 5.276 | | 1975 | 5.426 | | 1974 | 5.577 | | 1973 | 5.727 | | 1972 | 5.873 | | 1971 | 6.011 | | 1970 | 6.138 | | 1969 | 6.252 | | 1968 | 6.351 | | 1967 | 6.436 | | 1966 | 6.507 | | 1965 | 6.564 | | 1964 | 6.61 | | 1963 | 6.646 | | 1962 | 6.676 | | 1961 | 6.7 | | 1960 | 6.721 |
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