| Year | Value | | 2018 | 4.2 | | 2017 | 4.21 | | 2016 | 4.2 | | 2015 | 4.25 | | 2014 | 4.24 | | 2013 | 4.24 | | 2012 | 4.45 | | 2011 | 4.47 | | 2010 | 4.48 | | 2009 | 4.93 | | 2008 | 4.84 | | 2007 | 4.88 | | 2006 | 4.87 | | 2005 | 4.78 | | 2004 | 4.73 | | 2003 | 4.74 | | 2002 | 4.75 | | 2001 | 4.81 | | 2000 | 4.77 | | 1999 | 4.6999998093 | | 1998 | 4.8000001907 | | 1997 | 4.9000000954 | | 1996 | 4.9000000954 | | 1995 | 4.9000000954 | | 1994 | 4.9000000954 | | 1993 | 5 | | 1992 | 5 | | 1991 | 5 | | 1990 | 5.0999999046 | | 1989 | 5.0999999046326 | | 1988 | 5.1999998092651 | | 1987 | 5.1999998092651 | | 1986 | 5.3000001907349 | | 1985 | 5.5 | | 1980 | 6.1999998092651 | | 1970 | 6.1999998092651 | | 1960 | 5.8000001907349 |
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