| Year | Value | | 2017 | 2.43 | | 2016 | 2.48 | | 2015 | 2.43 | | 2014 | 2.42 | | 2013 | 2.49 | | 2012 | 2.5 | | 2011 | 2.98 | | 2010 | 1.2 | | 2007 | 2.9 | | 2006 | 2.9 | | 2005 | 2.4 | | 2003 | 1.8999999762 | | 1996 | 4.3899998665 | | 1993 | 4.5 | | 1990 | 4.522600174 | | 1985 | 3.2458000183105 | | 1975 | 5.0018000602722 | | 1970 | 5.912700176239 | | 1960 | 5.5118203163147 |
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