| Year | Value | | 2018 | 5.49 | | 2017 | 5.57 | | 2016 | 5.72 | | 2015 | 5.69 | | 2014 | 5.66 | | 2013 | 5.8 | | 2012 | 5.89 | | 2011 | 5.88 | | 2010 | 5.68 | | 2009 | 6.74 | | 2008 | 7.77 | | 2007 | 7.83 | | 2006 | 7.85 | | 2005 | 7.9 | | 2004 | 7.91 | | 2003 | 7.95 | | 2002 | 7.87 | | 2001 | 8.28 | | 2000 | 8.77 | | 1999 | 9.029999733 | | 1998 | 9.4099998474 | | 1997 | 9.8000001907 | | 1996 | 10.4399995804 | | 1995 | 11.1899995804 | | 1994 | 12.0551996231 | | 1993 | 12.2314996719 | | 1992 | 12.9406003952 | | 1991 | 13.6218996048 | | 1990 | 14.0754003525 | | 1989 | 14.023500442505 | | 1988 | 14.094900131226 | | 1987 | 14.239000320435 | | 1986 | 14.192899703979 | | 1985 | 14.30860042572 | | 1980 | 13.890999794006 |
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