| Year | Value | | 2014 | 6.06 | | 2013 | 6.72 | | 2012 | 7.01 | | 2011 | 7.11 | | 2010 | 7.29 | | 2009 | 7.61 | | 2008 | 7.71 | | 2007 | 7.72 | | 2006 | 7.78 | | 2005 | 7.76 | | 2004 | 7.77 | | 2003 | 7.7 | | 2002 | 7.02 | | 2001 | 6.97 | | 2000 | 7.19 | | 1999 | 7.25 | | 1998 | 8.1899995804 | | 1997 | 8.4499998093 | | 1996 | 10.3299999237 | | 1995 | 11.6499996185 | | 1994 | 12.1447000504 | | 1993 | 13.1665000916 | | 1992 | 13.4618997574 | | 1991 | 13.7089996338 | | 1990 | 13.6660995483 | | 1989 | 13.630000114441 | | 1988 | 13.544199943543 | | 1987 | 13.556099891663 | | 1986 | 13.560700416565 | | 1985 | 13.585700035095 | | 1984 | 13.512399673462 | | 1983 | 13.369400024414 | | 1982 | 13.311499595642 | | 1981 | 13.229100227356 | | 1980 | 13.093400001526 |
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