| Year | Value | | 2016 | 0.68061624899128 | | 2015 | 0.75426470755102 | | 2014 | 0.97038754824881 | | 2013 | 1.0027498038543 | | 2012 | 1.0139699363782 | | 2011 | 1.0058309624004 | | 2010 | 1.0986068200222 | | 2009 | 1.0533803677619 | | 2008 | 1.0223792955838 | | 2007 | 0.99329426715676 | | 2006 | 1.1668586038272 | | 2005 | 0.97885170521557 | | 2002 | 1.0975142482643 | | 2001 | 1.1474185704758 | | 2000 | 1.241390328742 | | 1991 | 1.5299500255899 | | 1985 | 1.6322521107374 | | 1981 | 1.5332549436634 | | 1980 | 1.5841831104896 | | 1975 | 0.84825752088665 | | 1970 | 0.88548728211785 | | 1960 | 0.79182440737612 |
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