| Year | Value | | 2018 | 3.53 | | 2017 | 3.6 | | 2016 | 3.68 | | 2015 | 3.76 | | 2014 | 3.84 | | 2013 | 3.86 | | 2012 | 3.97 | | 2011 | 4.19 | | 2010 | 4.3 | | 2009 | 4.52 | | 2008 | 4.64 | | 2007 | 4.86 | | 2006 | 5.03 | | 2005 | 5.16 | | 2004 | 5.25 | | 2003 | 4.99 | | 2002 | 4.99 | | 2001 | 3.82 | | 2000 | 3.8 | | 1999 | 3.9000000954 | | 1998 | 4 | | 1997 | 4 | | 1996 | 4 | | 1995 | 4 | | 1994 | 4.0999999046 | | 1993 | 4.1999998093 | | 1992 | 4.3000001907 | | 1991 | 4.4000000954 | | 1990 | 4.5999999046 | | 1989 | 4.8000001907349 | | 1988 | 5.3000001907349 | | 1987 | 5.6999998092651 | | 1986 | 5.9000000953674 |
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