| Year | Value | | 2017 | 3.4 | | 2016 | 3.43 | | 2015 | 3.42 | | 2014 | 3.42 | | 2013 | 3.41 | | 2012 | 3.46 | | 2011 | 3.51 | | 2010 | 3.57 | | 2009 | 3.76 | | 2008 | 3.78 | | 2007 | 3.8 | | 2006 | 3.8 | | 2005 | 3.83 | | 2004 | 4.2 | | 2003 | 4.3 | | 2002 | 4.4 | | 2001 | 4.4 | | 2000 | 4.52 | | 1989 | 5.0864000320435 | | 1981 | 5.7200999259949 | | 1980 | 5.6529998779297 | | 1970 | 5.3821001052856 | | 1960 | 4.5235600471497 |
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