| Year | Value | | 2017 | 1.38 | | 2016 | 1.36 | | 2015 | 1.1 | | 2013 | 1.15 | | 2012 | 1.07 | | 2011 | 1.07 | | 2010 | 1.93 | | 2009 | 1.93 | | 2008 | 1.86 | | 2007 | 1.88 | | 2006 | 1.88 | | 2005 | 2.19 | | 2004 | 2.19 | | 2003 | 2.19 | | 2002 | 2.19 | | 2001 | 2.28 | | 2000 | 2.38 | | 1996 | 2.6400001049 | | 1992 | 3.0604999065 | | 1990 | 2.6400001049 | | 1981 | 2.8750998973846 | | 1980 | 2.8494999408722 |
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