| Year | Value | | 2018 | 3.45 | | 2017 | 3.39 | | 2016 | 3.39 | | 2015 | 3.37 | | 2014 | 3.32 | | 2013 | 3.39 | | 2012 | 3.41 | | 2011 | 3.37 | | 2010 | 3.37 | | 2009 | 3.37 | | 2008 | 3.39 | | 2007 | 3.44 | | 2006 | 3.48 | | 2005 | 3.56 | | 2004 | 3.59 | | 2003 | 3.58 | | 2002 | 3.57 | | 2001 | 3.65 | | 2000 | 3.71 | | 1999 | 3.7999999523 | | 1998 | 3.9000000954 | | 1997 | 3.9000000954 | | 1996 | 4 | | 1995 | 3.9000000954 | | 1994 | 3.9000000954 | | 1993 | 3.9000000954 | | 1992 | 4 | | 1991 | 4.0999999046 | | 1990 | 4.0999999046 | | 1989 | 4.0999999046326 | | 1988 | 4 | | 1987 | 4 | | 1986 | 4 | | 1985 | 4 |
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