| Year | Value | | 2017 | 1.32 | | 2016 | 1.3 | | 2015 | 1.38 | | 2014 | 1.38 | | 2013 | 1.3 | | 2012 | 1.3 | | 2011 | 1.3 | | 2010 | 1.3 | | 2009 | 1.27 | | 2008 | 1.26 | | 2007 | 1.33 | | 2006 | 1.3 | | 2005 | 1.3 | | 2004 | 1.33 | | 2003 | 1.31 | | 2002 | 1.27 | | 2001 | 1.3 | | 2000 | 1.2 | | 1998 | 1.4500000477 | | 1990 | 1.6596000195 | | 1989 | 1.7138999700546 | | 1981 | 1.8931000232696 | | 1980 | 1.9464000463486 | | 1970 | 1.9509999752045 | | 1960 | 1.8580399751663 |
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