| Year | Value | | 2018 | 7.12 | | 2017 | 8.05 | | 2016 | 8.16 | | 2015 | 8.35 | | 2014 | 8.81 | | 2013 | 9.07 | | 2012 | 9.3 | | 2011 | 9.42 | | 2010 | 9.41 | | 2009 | 9.68 | | 2008 | 9.85 | | 2007 | 10.71 | | 2006 | 10.9 | | 2005 | 11.01 | | 2004 | 11.13 | | 2003 | 11.04 | | 2002 | 11.15 | | 2001 | 11.33 | | 2000 | 11.4 | | 1999 | 10.8500003815 | | 1998 | 11.1099996567 | | 1997 | 11.3400001526 | | 1996 | 11.6300001144 | | 1995 | 11.8699998856 | | 1994 | 11.9611997604 | | 1993 | 12.1958999634 | | 1992 | 12.2342996597 | | 1991 | 12.7019996643 | | 1990 | 13.0552997589 | | 1989 | 13.186200141907 | | 1988 | 13.175200462341 | | 1987 | 13.117600440979 | | 1986 | 13.046299934387 | | 1985 | 12.981599807739 |
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