| Year | Value | | 2016 | 1.39 | | 2015 | 1.57 | | 2014 | 1.55 | | 2013 | 1.51 | | 2012 | 1.5 | | 2011 | 1.62 | | 2010 | 1.59 | | 2009 | 1.6 | | 2008 | 1.5 | | 2007 | 1.5 | | 2003 | 1.7 | | 2002 | 1.5 | | 1996 | 1.5499999523 | | 1993 | 1.6000000238 | | 1990 | 1.6440999508 | | 1980 | 1.9071999788284 | | 1975 | 2.5462999343872 | | 1970 | 2.349100112915 | | 1960 | 1.8706915378571 |
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