| Year | Value | | 2017 | 0.53 | | 2016 | 0.48 | | 2015 | 0.58 | | 2014 | 0.52 | | 2013 | 0.49 | | 2012 | 0.49 | | 2011 | 0.63 | | 2010 | 0.47 | | 2009 | 0.44 | | 2008 | 0.41 | | 2007 | 0.41 | | 2006 | 0.42 | | 2005 | 0.41 | | 2003 | 0.8999999762 | | 2002 | 0.63 | | 2001 | 0.64 | | 2000 | 0.65 | | 1991 | 0.7870000005 | | 1987 | 0.71820002794266 | | 1985 | 0.76920002698898 | | 1981 | 0.76940000057221 | | 1980 | 0.76969999074936 | | 1975 | 0.58819997310638 | | 1970 | 0.60449999570847 | | 1960 | 0.4589869081974 |
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