| Year | Value | | 2017 | 2.04 | | 2016 | 2.04 | | 2015 | 2.04 | | 2014 | 2.04 | | 2013 | 2.2 | | 2012 | 2.2 | | 2011 | 1.85 | | 2010 | 2 | | 2009 | 1.8 | | 2008 | 1.8 | | 2007 | 1.9 | | 2006 | 1.9 | | 2005 | 1.9 | | 2004 | 2.1 | | 2003 | 2.2 | | 2002 | 2.2 | | 2001 | 2.3 | | 2000 | 2.4 | | 1997 | 2.7599999905 | | 1989 | 2.9853999614716 | | 1980 | 4.144700050354 | | 1970 | 4.8347001075745 | | 1960 | 6.7985610961914 |
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