| Year | Value | | 2018 | 3.61 | | 2017 | 3.28 | | 2016 | 3.97 | | 2015 | 4.35 | | 2014 | 4.53 | | 2013 | 4.87 | | 2012 | 5.3 | | 2011 | 5.52 | | 2010 | 5.85 | | 2009 | 6.25 | | 2008 | 6.57 | | 2007 | 6.73 | | 2006 | 6.99 | | 2005 | 7.05 | | 2004 | 7.1 | | 2003 | 7.24 | | 2002 | 7.35 | | 2001 | 7.46 | | 2000 | 7.54 | | 1999 | 7.5999999046 | | 1998 | 7.8000001907 | | 1997 | 7.9000000954 | | 1996 | 9.1999998093 | | 1995 | 9.3000001907 | | 1994 | 10 | | 1993 | 10 | | 1992 | 11 | | 1991 | 11.1999998093 | | 1990 | 12.5 | | 1985 | 14 | | 1980 | 15.60000038147 | | 1970 | 15.10000038147 |
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