| Year | Value | | 2017 | 3.2 | | 2016 | 3.7 | | 2015 | 3.7 | | 2014 | 3.7 | | 2013 | 3.7 | | 2012 | 3.7 | | 2011 | 3.7 | | 2010 | 3.7 | | 2009 | 3.7 | | 2008 | 3.7 | | 2007 | 3.7 | | 2006 | 3.7 | | 2005 | 3.4 | | 2004 | 3.9 | | 2003 | 3.9 | | 2002 | 3.9 | | 2001 | 3.85 | | 2000 | 4.12 | | 1997 | 4.3000001907 | | 1990 | 4.1729998589 | | 1981 | 4.8255000114441 | | 1970 | 3.8136999607086 | | 1960 | 2.7553744316101 |
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