| Year | Value | | 2017 | 1.47 | | 2016 | 1.49 | | 2015 | 1.56 | | 2014 | 1.58 | | 2013 | 1.71 | | 2012 | 1.65 | | 2011 | 1.77 | | 2010 | 1.77 | | 2009 | 1.9 | | 2008 | 2.02 | | 2007 | 2.02 | | 2006 | 2.1 | | 2005 | 2.1 | | 2004 | 2.2 | | 2003 | 2.04 | | 2002 | 2.04 | | 2001 | 2.1 | | 2000 | 2.18 | | 1998 | 2.2000000477 | | 1990 | 2.1087999344 | | 1989 | 2.5072000026703 | | 1981 | 1.6075999736786 | | 1980 | 1.6203000545502 | | 1960 | 0.33512544631958 |
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