| Year | Value | | 2018 | 1.71 | | 2017 | 1.7 | | 2016 | 1.68 | | 2015 | 1.61 | | 2014 | 1.59 | | 2013 | 1.54 | | 2012 | 1.48 | | 2011 | 1.45 | | 2010 | 1.48 | | 2007 | 1 | | 2006 | 1 | | 2004 | 1.2 | | 2003 | 1.1000000238 | | 1996 | 1.4600000381 | | 1993 | 1.2999999523 | | 1990 | 1.3667000532 | | 1989 | 1.2510999441147 | | 1980 | 1.5641000270843 | | 1970 | 2.0973000526428 | | 1960 | 2.5755472183227 |
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