| Year | Value | | 2018 | 3.17 | | 2017 | 3.28 | | 2016 | 3.41 | | 2015 | 3.49 | | 2013 | 3.61 | | 2012 | 3.75 | | 2011 | 3.84 | | 2010 | 4.05 | | 2009 | 4.2 | | 2008 | 4.27 | | 2007 | 4.31 | | 2006 | 4.34 | | 2005 | 4.5 | | 2004 | 4.56 | | 2003 | 4.58 | | 2002 | 4.69 | | 2001 | 4.75 | | 2000 | 4.92 | | 1999 | 5 | | 1998 | 5.0999999046 | | 1997 | 5.1999998093 | | 1996 | 5.1999998093 | | 1995 | 5.3000001907 | | 1994 | 5.4000000954 | | 1993 | 5.5999999046 | | 1992 | 5.6999998093 | | 1991 | 5.6999998093 | | 1990 | 5.8000001907 |
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