| Year | Value | | 2017 | 0.74 | | 2015 | 0.67 | | 2013 | 0.82 | | 2012 | 0.82 | | 2011 | 0.84 | | 2010 | 0.72 | | 2009 | 0.73 | | 2008 | 0.73 | | 2007 | 0.73 | | 2006 | 0.73 | | 2005 | 0.72 | | 2004 | 0.71 | | 2003 | 0.73 | | 2002 | 0.73 | | 2001 | 0.74 | | 2000 | 0.81 | | 1990 | 1.0880000591 | | 1981 | 0.87019997835159 | | 1980 | 0.89069998264313 | | 1975 | 0.9932000041008 | | 1970 | 1.0426000356674 | | 1960 | 1.0065009593964 |
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