| Year | Value | | 2017 | 3.57 | | 2016 | 3.58 | | 2015 | 3.78 | | 2014 | 3.8 | | 2013 | 3.81 | | 2012 | 3.58 | | 2011 | 3.6 | | 2009 | 2.4 | | 2008 | 2.6 | | 2007 | 2.6 | | 2005 | 4.1 | | 2003 | 5.6999998093 | | 2002 | 6.9 | | 2000 | 6.2 | | 1996 | 5.2699999809 | | 1995 | 5.5 | | 1993 | 8.1778001785 | | 1990 | 8 | | 1980 | 5.771399974823 | | 1970 | 7.2536997795105 | | 1960 | 6.222222328186 |
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