| Year | Value | | 2017 | 1.25 | | 2016 | 1.2 | | 2015 | 1.2 | | 2014 | 1.2 | | 2013 | 1.4 | | 2012 | 1.2 | | 2011 | 1.2 | | 2010 | 1.23 | | 2009 | 1.4 | | 2008 | 2.52 | | 2007 | 2.52 | | 2006 | 2.52 | | 2005 | 2.43 | | 2004 | 2.36 | | 2003 | 2.36 | | 2002 | 2.36 | | 2001 | 2.42 | | 2000 | 2.36 | | 1997 | 1.6499999762 | | 1989 | 2.3002998828888 | | 1981 | 2.9395999908447 | | 1970 | 5.5314998626709 |
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