| Year | Value | | 2018 | 5.91 | | 2017 | 5.98 | | 2016 | 6.06 | | 2015 | 6.13 | | 2014 | 6.2 | | 2013 | 6.28 | | 2012 | 6.34 | | 2011 | 6.36 | | 2010 | 6.43 | | 2009 | 6.66 | | 2008 | 6.9 | | 2007 | 7.06 | | 2006 | 7.11 | | 2005 | 7.22 | | 2004 | 7.39 | | 2003 | 7.55 | | 2002 | 7.71 | | 2001 | 7.83 | | 2000 | 7.97 | | 1999 | 8.3000001907 | | 1998 | 8.3999996185 | | 1997 | 8.6000003815 | | 1996 | 8.8000001907 | | 1995 | 8.8999996185 | | 1994 | 9.1000003815 | | 1993 | 9.3000001907 | | 1992 | 9.3999996185 | | 1991 | 9.6000003815 | | 1990 | 9.6999998093 | | 1989 | 9.8999996185303 | | 1988 | 10.10000038147 | | 1987 | 10.199999809265 | | 1986 | 10.300000190735 | | 1985 | 10.5 | | 1984 | 10.699999809265 | | 1983 | 10.800000190735 | | 1982 | 11 | | 1981 | 11.10000038147 | | 1980 | 11.10000038147 | | 1979 | 11.10000038147 | | 1978 | 10.89999961853 | | 1977 | 10.800000190735 | | 1976 | 10.60000038147 | | 1975 | 10.60000038147 | | 1974 | 10.5 |
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