| Year | Value | | 2017 | 2.18 | | 2016 | 2.18 | | 2015 | 2.18 | | 2014 | 2.18 | | 2013 | 2.12 | | 2012 | 2.12 | | 2011 | 2.09 | | 2010 | 2.09 | | 2009 | 2.09 | | 2008 | 2 | | 2007 | 1.76 | | 2006 | 1.98 | | 2005 | 1.76 | | 2004 | 2.05 | | 2003 | 1.71 | | 2002 | 1.71 | | 2001 | 1.73 | | 2000 | 1.73 | | 1997 | 1.7000000477 | | 1996 | 2 | | 1994 | 1.761500001 | | 1992 | 2.8573000431 | | 1991 | 2.0123000145 | | 1990 | 1.9167000055 | | 1989 | 1.9657000303268 | | 1980 | 2.1257998943329 | | 1975 | 2.5 | | 1970 | 2.4442999362946 | | 1960 | 2.4676616191864 |
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