| Year | Value | | 2017 | 1.56 | | 2016 | 1.7 | | 2015 | 1.5 | | 2014 | 1.5 | | 2013 | 1.5 | | 2012 | 1.5 | | 2011 | 1.74 | | 2010 | 1.74 | | 2009 | 1.38 | | 2008 | 1.38 | | 2007 | 1.72 | | 2006 | 1.72 | | 2005 | 1.72 | | 2004 | 1.63 | | 2003 | 1.63 | | 2002 | 1.63 | | 2001 | 1.59 | | 2000 | 1.59 | | 1996 | 1.6000000238 | | 1990 | 1.4330999851 | | 1988 | 1.4837000370026 | | 1987 | 1.5104999542236 | | 1981 | 1.5155999660492 | | 1980 | 1.4805999994278 | | 1970 | 1.3796000480652 | | 1960 | 0.92567503452301 |
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