| Year | Value | | 2017 | 2.73 | | 2016 | 2.73 | | 2015 | 3.37 | | 2014 | 2.85 | | 2013 | 2.86 | | 2012 | 3.45 | | 2011 | 3.45 | | 2010 | 3.45 | | 2009 | 3.45 | | 2008 | 3.43 | | 2007 | 3.43 | | 2006 | 3.6 | | 2005 | 3.6 | | 2004 | 3 | | 2003 | 3 | | 2002 | 3 | | 2001 | 3 | | 2000 | 3.07 | | 1997 | 2.7000000477 | | 1992 | 3.0827000141 | | 1990 | 1.6505999565 | | 1970 | 4.0987000465393 | | 1960 | 4.5813393592834 |
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