| Year | Value | | 2017 | 2.2868642639142 | | 2016 | 2.1114291710292 | | 2015 | 2.070733461122 | | 2014 | 2.1588255095628 | | 2013 | 2.169827618985 | | 2012 | 2.1322084653655 | | 2011 | 2.0380039229225 | | 2010 | 1.9670439309799 | | 2009 | 1.8742101997316 | | 2008 | 1.8739434174281 | | 2007 | 1.8894003124213 | | 2006 | 1.8830118376412 | | 2005 | 1.8407963961593 | | 2003 | 2.0217373394689 | | 2002 | 1.8537098241342 | | 2001 | 1.897808111999 | | 2000 | 2.0048633661535 | | 1991 | 2.5927079085551 | | 1990 | 2.9236476942355 | | 1985 | 2.7139597579279 | | 1980 | 1.9903562632269 | | 1975 | 1.5443432477857 | | 1970 | 1.3770684783812 |
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