| Year | Value | | 2018 | 6.62 | | 2017 | 6.63 | | 2016 | 6.66 | | 2015 | 6.67 | | 2014 | 6.68 | | 2013 | 6.7 | | 2012 | 6.93 | | 2011 | 7.06 | | 2010 | 7.3 | | 2009 | 7.14 | | 2008 | 7.21 | | 2007 | 7.33 | | 2006 | 7.45 | | 2005 | 7.57 | | 2004 | 7.65 | | 2003 | 7.73 | | 2002 | 7.76 | | 2001 | 7.77 | | 2000 | 7.8 | | 1994 | 10.1999998093 | | 1993 | 10.3999996185 | | 1992 | 10.6999998093 | | 1991 | 11.1000003815 | | 1990 | 11.3000001907 | | 1985 | 11.39999961853 | | 1980 | 11.300000190735 |
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