| Year | Value | | 2017 | 1.59 | | 2016 | 1.59 | | 2015 | 1.55 | | 2014 | 1.56 | | 2013 | 1.51 | | 2012 | 1.52 | | 2011 | 1.53 | | 2010 | 1.5 | | 2009 | 1.6 | | 2008 | 1.5 | | 2007 | 1.2 | | 2004 | 0.9 | | 2003 | 1.3999999762 | | 1996 | 1.4700000286 | | 1994 | 1.3571000099 | | 1993 | 1.2999999523 | | 1992 | 1.5 | | 1990 | 1.4132000208 | | 1985 | 1.6844999790192 | | 1975 | 2.5 | | 1970 | 2.1728000640869 | | 1960 | 2.4114389419556 |
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