| Year | Value | | 2017 | 2.3271770501941 | | 2016 | 2.1542402554653 | | 2015 | 2.113799821946 | | 2014 | 2.200055141238 | | 2013 | 2.2228049416427 | | 2012 | 2.1262251318743 | | 2011 | 2.0830960832916 | | 2010 | 1.9532991720177 | | 2009 | 1.9265887827938 | | 2008 | 1.9337098298804 | | 2007 | 1.9485189690116 | | 2006 | 1.9467636423761 | | 2005 | 1.9041054232284 | | 2003 | 2.0863072142715 | | 2002 | 1.8606983321318 | | 2001 | 1.9320281814592 | | 2000 | 2.0744237645035 | | 1991 | 2.6803290163073 | | 1990 | 2.997047108616 | | 1985 | 2.8163689381469 | | 1980 | 2.098187589755 | | 1975 | 1.5458064020278 | | 1970 | 1.4350662183052 |
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