| Year | Value | | 2017 | 0.58805803632625 | | 2016 | 0.52709465020209 | | 2015 | 0.59826037647978 | | 2014 | 0.53671348257256 | | 2013 | 0.50560772976823 | | 2012 | 0.54431929064589 | | 2011 | 0.62071915254459 | | 2010 | 0.52861740464131 | | 2009 | 0.46165753829292 | | 2008 | 0.43508032902909 | | 2007 | 0.48698961647001 | | 2006 | 0.5198512367456 | | 2005 | 0.43407533994052 | | 2003 | 0.86339921523235 | | 2002 | 0.63135154785614 | | 2001 | 0.63145379912669 | | 2000 | 0.68096810860496 | | 1991 | 0.71715242476843 | | 1987 | 0.70253752280059 | | 1985 | 0.70595344807608 | | 1981 | 0.72949219565369 | | 1980 | 0.72584475053949 | | 1975 | 0.6257610850332 | | 1970 | 0.58275593771438 | | 1960 | 0.50655663705247 |
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