| Year | Value | | 2017 | 1.74 | | 2016 | 1.89 | | 2015 | 1.95 | | 2014 | 2.03 | | 2013 | 2.08 | | 2012 | 2.08 | | 2011 | 1.7 | | 2010 | 1.77 | | 2009 | 1.9 | | 2008 | 1.97 | | 2007 | 2.74 | | 2006 | 2.74 | | 2005 | 2.81 | | 2004 | 2.81 | | 2003 | 2.81 | | 2002 | 2.86 | | 2001 | 2.82 | | 2000 | 2.75 | | 1997 | 2.9000000954 | | 1989 | 3.3394999504089 | | 1980 | 3.3143999576569 | | 1970 | 4.3428997993469 | | 1960 | 4.7114095687866 |
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