| Year | Value | | 2017 | 1.4 | | 2016 | 1.46 | | 2015 | 1.47 | | 2014 | 1.5 | | 2013 | 1.52 | | 2012 | 1.53 | | 2011 | 1.55 | | 2010 | 1.54 | | 2009 | 1.51 | | 2008 | 1.54 | | 2007 | 1.47 | | 2006 | 1.47 | | 2005 | 1.28 | | 2004 | 1.49 | | 2003 | 1.49 | | 2002 | 1.38 | | 2001 | 1.4 | | 2000 | 1.4 | | 1999 | 1.3999999762 | | 1998 | 1.5 | | 1996 | 1.5 | | 1990 | 1.0865000486 | | 1981 | 1.1457999944687 | | 1980 | 1.1045000553131 | | 1975 | 1 | | 1970 | 0.99339997768402 | | 1960 | 1.0767375230789 |
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