| Year | Value | | 2018 | 6.43 | | 2017 | 6.56 | | 2016 | 6.69 | | 2015 | 6.97 | | 2014 | 7.26 | | 2013 | 7.31 | | 2012 | 7.45 | | 2011 | 7.43 | | 2010 | 7.16 | | 2009 | 7.18 | | 2008 | 7.18 | | 2007 | 7.19 | | 2006 | 7.16 | | 2005 | 7.28 | | 2004 | 7.57 | | 2003 | 7.85 | | 2002 | 8.1 | | 2001 | 8.33 | | 2000 | 8.83 | | 1999 | 9.8500003815 | | 1998 | 10.029999733 | | 1997 | 10.1899995804 | | 1996 | 10.8800001144 | | 1995 | 11.0900001526 | | 1994 | 11.2838001251 | | 1993 | 11.9105997086 | | 1992 | 12.0453996658 | | 1991 | 12.4653997421 | | 1990 | 12.4871997833 | | 1989 | 12.635800361633 | | 1988 | 12.805399894714 | | 1987 | 12.802900314331 | | 1986 | 12.769200325012 | | 1985 | 12.776000022888 | | 1980 | 12.06639957428 |
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