| Year | Value | | 2018 | 7.27 | | 2017 | 7.37 | | 2016 | 7.42 | | 2015 | 7.54 | | 2014 | 7.58 | | 2013 | 7.64 | | 2012 | 7.67 | | 2011 | 7.68 | | 2010 | 7.65 | | 2009 | 7.68 | | 2008 | 7.69 | | 2007 | 7.75 | | 2006 | 7.66 | | 2005 | 7.69 | | 2004 | 7.73 | | 2003 | 7.73 | | 2002 | 7.81 | | 2001 | 7.85 | | 2000 | 7.95 | | 1999 | 8.8999996185 | | 1998 | 9 | | 1997 | 9.1999998093 | | 1996 | 9.3000001907 | | 1995 | 9.3999996185 | | 1994 | 9.5 | | 1993 | 9.3999996185 | | 1992 | 9.6999998093 | | 1991 | 9.8999996185 | | 1990 | 10.1999998093 | | 1989 | 10.39999961853 | | 1988 | 10.60000038147 | | 1987 | 10.699999809265 | | 1986 | 10.800000190735 | | 1985 | 10.89999961853 | | 1980 | 11.199999809265 | | 1970 | 10.800000190735 | | 1960 | 10.800000190735 |
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