| Year | Value | | 2017 | 2.24 | | 2016 | 2.23 | | 2015 | 2.2 | | 2014 | 2.65 | | 2013 | 2.2 | | 2012 | 2.09 | | 2011 | 2.14 | | 2010 | 2.2 | | 2009 | 2.17 | | 2008 | 2.21 | | 2007 | 2.2 | | 2006 | 2.3 | | 2005 | 2.25 | | 2004 | 2.24 | | 2003 | 2.24 | | 2002 | 2.24 | | 2000 | 2.3 | | 1997 | 2.2999999523 | | 1990 | 2.4964001179 | | 1981 | 1.4581999778748 | | 1970 | 1.1813999414444 | | 1960 | 0.77472394704819 |
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