| Year | Value | | 2017 | 2.3450804506311 | | 2016 | 2.1807927720849 | | 2015 | 2.1575145787042 | | 2014 | 2.1884790035645 | | 2013 | 2.2087286734365 | | 2012 | 2.1032746962227 | | 2011 | 1.9943507492754 | | 2010 | 1.9655562204784 | | 2009 | 1.9133525833445 | | 2008 | 1.9410708096787 | | 2007 | 1.926900194373 | | 2006 | 1.9826481350913 | | 2005 | 1.9052157677393 | | 2003 | 2.0573464602898 | | 2002 | 1.8215921279708 | | 2001 | 1.926527436153 | | 2000 | 2.0358246350932 | | 1991 | 2.6094967336034 | | 1990 | 3.0654739055003 | | 1985 | 2.7143036721522 | | 1981 | 2.0062366307885 | | 1980 | 2.0301825512285 | | 1975 | 1.5564577338739 | | 1970 | 1.3849500755996 |
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