| Year | Value | | 2014 | 7.46 | | 2013 | 8.8 | | 2012 | 8.91 | | 2011 | 9.05 | | 2010 | 9.38 | | 2009 | 9.4 | | 2008 | 8.75 | | 2007 | 8.72 | | 2006 | 8.73 | | 2005 | 8.68 | | 2004 | 8.73 | | 2003 | 8.81 | | 2002 | 8.89 | | 2001 | 8.79 | | 2000 | 8.82 | | 1999 | 8.9499998093 | | 1998 | 9.0399999619 | | 1997 | 9.3699998856 | | 1996 | 10.8400001526 | | 1995 | 11.8900003433 | | 1994 | 12.3022003174 | | 1993 | 12.470700264 | | 1992 | 12.7290000916 | | 1991 | 12.9690999985 | | 1990 | 13.0130996704 | | 1989 | 13.006400108337 | | 1988 | 12.930399894714 | | 1987 | 12.850600242615 | | 1986 | 12.787599563599 | | 1985 | 12.694999694824 | | 1984 | 12.590999603272 | | 1983 | 12.444800376892 | | 1982 | 12.357799530029 | | 1981 | 12.253299713135 | | 1980 | 12.143500328064 |
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