| Year | Value | | 2016 | 0.842 | | 2015 | 0.932 | | 2014 | 0.976 | | 2013 | 0.994 | | 2012 | 1.016 | | 2011 | 0.997 | | 2010 | 0.971 | | 2009 | 0.974 | | 2008 | 0.993 | | 2007 | 1.011 | | 2006 | 1.001 | | 2005 | 1.029 | | 2004 | 0.963 | | 2003 | 1.048 | | 2002 | 1.058 | | 2001 | 1.016 | | 2000 | 0.953 | | 1997 | 1 | | 1991 | 1.315 | | 1985 | 1.219 | | 1980 | 1.087 | | 1970 | 0.429 | | 1960 | 0.45 |
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