| Year | Value | | 2015 | 4.2 | | 2014 | 4.16 | | 2013 | 4.06 | | 2012 | 4.05 | | 2011 | 3.74 | | 2010 | 3.73 | | 2009 | 3.72 | | 2008 | 3.82 | | 2007 | 4.07 | | 2006 | 4.42 | | 2005 | 4.46 | | 2004 | 4.44 | | 2003 | 4.42 | | 2002 | 4.35 | | 2001 | 5.03 | | 2000 | 6.44 | | 1999 | 6.1999998093 | | 1998 | 6.6599998474 | | 1997 | 6.75 | | 1996 | 7.1300001144 | | 1995 | 7.6399998665 | | 1994 | 7.765999794 | | 1993 | 8.2146997452 | | 1992 | 8.3676996231 | | 1991 | 8.529299736 | | 1990 | 9.0947999954 | | 1989 | 9.0357999801636 | | 1988 | 9.1904001235962 | | 1987 | 8.996600151062 | | 1986 | 9.0242004394531 | | 1985 | 8.6991996765137 | | 1984 | 8.6600999832153 | | 1983 | 8.5481004714966 | | 1982 | 8.6828002929688 | | 1981 | 8.5783996582031 | | 1980 | 8.3954000473022 |
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