| Year | Value | | 2017 | 2.4097186006179 | | 2016 | 2.2981073727105 | | 2015 | 2.4886963260572 | | 2013 | 2.25763750648 | | 2012 | 2.3080056480848 | | 2010 | 2.3139158492592 | | 2009 | 2.3080387488329 | | 2007 | 2.1987450820454 | | 2006 | 2.5639092354436 | | 2005 | 2.4514196128619 | | 2003 | 2.5574171954534 | | 2001 | 2.5213155910321 | | 1996 | 3.4980738259636 | | 1995 | 3.1027503586882 | | 1993 | 2.8389571249227 | | 1990 | 3.6382962216228 | | 1975 | 4.9903011852086 | | 1970 | 5.0051579752175 | | 1960 | 5.0454388889148 |
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