| Year | Value | | 2017 | 1.72 | | 2016 | 1.78 | | 2015 | 1.69 | | 2013 | 1.66 | | 2012 | 1.67 | | 2010 | 1.76 | | 2009 | 1.7 | | 2007 | 1.7 | | 2006 | 2 | | 2005 | 1.7 | | 2004 | 1.8 | | 2003 | 1.3999999762 | | 2002 | 1.5 | | 2001 | 1.5 | | 1996 | 2.1199998856 | | 1995 | 2.1 | | 1993 | 2.1003000736 | | 1990 | 2.2000000477 | | 1989 | 2.4000999927521 | | 1985 | 3.333300113678 | | 1975 | 4.999499797821 | | 1970 | 4.1048998832703 | | 1960 | 4.1454882621765 |
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