| Year | Value | | 2017 | 8 | | 2016 | 8.06 | | 2015 | 8.13 | | 2014 | 8.23 | | 2013 | 8.28 | | 2012 | 8.34 | | 2011 | 8.38 | | 2010 | 8.25 | | 2009 | 8.24 | | 2008 | 8.21 | | 2007 | 8.24 | | 2006 | 8.3 | | 2005 | 8.47 | | 2004 | 8.58 | | 2003 | 8.74 | | 2002 | 8.87 | | 2001 | 9.01 | | 2000 | 9.12 | | 1999 | 9.1999998093 | | 1998 | 9.3000001907 | | 1997 | 9.3999996185 | | 1996 | 9.6000003815 | | 1995 | 9.6999998093 | | 1994 | 9.6999998093 | | 1993 | 9.6999998093 | | 1992 | 9.8999996185 | | 1991 | 10.1000003815 | | 1990 | 10.3999996185 | | 1985 | 11 | | 1980 | 11.5 | | 1970 | 11.300000190735 | | 1960 | 10.5 |
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