| Year | Value | | 2018 | 5.7 | | 2017 | 5.82 | | 2016 | 5.78 | | 2015 | 5.75 | | 2014 | 5.79 | | 2013 | 5.8 | | 2012 | 5.91 | | 2011 | 6.05 | | 2010 | 6.46 | | 2009 | 6.54 | | 2008 | 6.59 | | 2007 | 6.78 | | 2006 | 6.73 | | 2005 | 6.79 | | 2004 | 6.91 | | 2003 | 7.25 | | 2002 | 7.57 | | 2001 | 7.67 | | 2000 | 7.86 | | 1999 | 8 | | 1998 | 8 | | 1997 | 8.1000003815 | | 1996 | 8.3000001907 | | 1994 | 7.0992999077 | | 1993 | 7.8740000725 | | 1992 | 7.5609002113 | | 1991 | 7.5778999329 | | 1990 | 7.4482002258 | | 1989 | 8.81631 | | 1988 | 8.81316 | | 1987 | 8.95051 | | 1986 | 8.94222 | | 1985 | 8.95471 |
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