| Year | Value | | 2017 | 0.64 | | 2016 | 0.64 | | 2015 | 0.66 | | 2014 | 0.63 | | 2013 | 0.65 | | 2012 | 0.62 | | 2011 | 0.62 | | 2010 | 0.8 | | 2009 | 0.8 | | 2008 | 0.7 | | 2002 | 1 | | 1996 | 1.0599999428 | | 1993 | 0.8000000119 | | 1990 | 1.0067000389 | | 1985 | 1.3094999790192 | | 1980 | 1.3241000175476 | | 1975 | 1.7036999464035 | | 1970 | 1.7531000375748 | | 1960 | 1.6462513208389 |
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