| Year | Value | | 2018 | 6.54 | | 2017 | 6.62 | | 2016 | 6.64 | | 2015 | 6.63 | | 2014 | 6.63 | | 2013 | 6.61 | | 2012 | 6.63 | | 2011 | 6.63 | | 2010 | 6.61 | | 2009 | 6.65 | | 2008 | 6.62 | | 2007 | 6.42 | | 2006 | 6.47 | | 2005 | 6.52 | | 2004 | 6.67 | | 2003 | 6.68 | | 2002 | 5.5999999046 | | 2000 | 4.9000000954 | | 1999 | 5.0999999046 | | 1998 | 5.3000001907 | | 1997 | 5.4000000954 | | 1996 | 5.5 | | 1995 | 5.5 | | 1994 | 5.5999999046 | | 1993 | 5.5999999046 | | 1992 | 5.5999999046 | | 1991 | 5.6999998093 | | 1990 | 5.6999998093 | | 1985 | 5.6999998092651 | | 1980 | 5.5999999046326 | | 1970 | 5.1999998092651 | | 1960 | 4.5999999046326 |
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