| Year | Value | | 2017 | 1.9001282426276 | | 2016 | 1.91564327168 | | 2015 | 1.8877198461498 | | 2014 | 1.9168750387279 | | 2013 | 1.9982672337691 | | 2012 | 1.9733779254537 | | 2011 | 1.9236755186186 | | 2010 | 2.0258769519025 | | 2009 | 1.8453626622577 | | 2008 | 1.8795964623285 | | 2007 | 1.7620384040952 | | 2006 | 1.9054824886316 | | 2005 | 2.1947012166971 | | 2003 | 1.8287138921036 | | 1996 | 2.272527102126 | | 1993 | 2.5721417504619 | | 1990 | 2.4984866132033 | | 1970 | 3.4701728735978 | | 1960 | 3.42807919637 |
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