| Year | Value | | 2017 | 3.02 | | 2016 | 3.03 | | 2015 | 3.01 | | 2014 | 2.99 | | 2013 | 2.7 | | 2012 | 2.72 | | 2010 | 2.14 | | 2009 | 2.6 | | 2008 | 2.5 | | 2007 | 2.7 | | 2006 | 2.7 | | 2005 | 2.6 | | 2003 | 3.3 | | 2001 | 3.4000000954 | | 2000 | 3.3 | | 1996 | 5.1100001335 | | 1995 | 3.8 | | 1993 | 3.2425999641 | | 1990 | 4 | | 1986 | 5.0373001098633 | | 1975 | 4.9851999282837 | | 1970 | 4.9433999061584 | | 1960 | 5.5895609855652 |
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