| Year | Value | | 2017 | 0.63 | | 2016 | 0.63 | | 2015 | 0.6 | | 2014 | 0.6 | | 2013 | 0.6 | | 2012 | 0.6 | | 2011 | 0.6 | | 2010 | 0.6 | | 2009 | 0.6 | | 2008 | 0.6 | | 2007 | 1 | | 2005 | 0.7 | | 2004 | 0.68 | | 2003 | 0.68 | | 2002 | 0.69 | | 2001 | 0.66 | | 2000 | 0.67 | | 1993 | 0.6514999866 | | 1992 | 0.6446999907 | | 1991 | 0.6521999836 | | 1990 | 0.6399999857 | | 1989 | 0.60430002212524 | | 1986 | 0.59950000047684 | | 1981 | 0.5692999958992 | | 1980 | 0.57309997081757 | | 1975 | 0.52630001306534 | | 1970 | 0.52079999446869 | | 1960 | 0.55726158618927 |
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