| Year | Value | | 2017 | 0.93 | | 2016 | 0.93 | | 2015 | 0.93 | | 2014 | 0.93 | | 2013 | 0.95 | | 2012 | 0.96 | | 2011 | 1.14 | | 2010 | 0.8 | | 2008 | 0.9 | | 2007 | 0.9 | | 2006 | 1 | | 2005 | 0.9 | | 2004 | 0.9 | | 2003 | 0.8999999762 | | 1996 | 1.4800000191 | | 1993 | 1.7999999523 | | 1990 | 1.8213000298 | | 1989 | 1.8269000053406 | | 1985 | 2.4059998989105 | | 1975 | 2.4119000434876 | | 1970 | 2.2802999019623 | | 1960 | 2.1582360267639 |
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