Year | Value | 2017 | 1.8302660997014 | 2016 | 1.8476469433979 | 2015 | 1.8211574844476 | 2014 | 1.851575842725 | 2013 | 1.931474357566 | 2012 | 1.9061583191863 | 2011 | 1.8597709213466 | 2010 | 1.9390952503576 | 2009 | 1.7424535044465 | 2008 | 1.7670542310612 | 2007 | 1.6911210197624 | 2006 | 1.8165130349134 | 2005 | 2.1162289277397 | 2003 | 1.746811625167 | 2000 | 2.38462634708 | 1996 | 2.1905617828417 | 1993 | 2.4888895242818 | 1990 | 2.420397245694 | 1970 | 3.4124396259908 | 1960 | 3.3814864770032 |
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