| Year | Value | | 2017 | 1.9421031027414 | | 2016 | 1.9581013137206 | | 2015 | 1.9317776965891 | | 2014 | 1.9628574506478 | | 2013 | 1.9820736080236 | | 2012 | 1.9559208504611 | | 2011 | 1.9591109619708 | | 2010 | 2.0250782772657 | | 2009 | 1.8668662869329 | | 2008 | 1.8529678345259 | | 2007 | 1.7543638270786 | | 2006 | 1.8667801760181 | | 2005 | 2.1819302344737 | | 2003 | 1.849965089563 | | 2002 | 1.9285163312922 | | 2000 | 2.364984718365 | | 1996 | 2.2562939623819 | | 1993 | 2.5336493095932 | | 1990 | 2.4397362565958 | | 1970 | 3.403419393342 | | 1960 | 3.3534103953996 |
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