| Year | Value | | 2017 | 1.88 | | 2016 | 1.62 | | 2015 | 1.86 | | 2012 | 1.9 | | 2011 | 1.79 | | 2010 | 1.8 | | 2009 | 1.8 | | 2007 | 1.76 | | 2006 | 1.9 | | 2005 | 1.8 | | 2001 | 1.8 | | 1996 | 2.0099999905 | | 1995 | 2.0299999714 | | 1994 | 2.0215001106 | | 1993 | 2.0295000076 | | 1992 | 2.0606999397 | | 1991 | 2.0938999653 | | 1990 | 2.1305999756 | | 1989 | 2.3394999504089 | | 1987 | 2.4779000282288 | | 1985 | 2.5576000213623 | | 1981 | 2.2783000469208 | | 1975 | 3.3466999530792 | | 1970 | 3.4697000980377 | | 1960 | 3.728132724762 |
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