| Year | Value | | 2017 | 6.89 | | 2016 | 6.84 | | 2015 | 6.79 | | 2014 | 6.71 | | 2013 | 6.67 | | 2012 | 6.6 | | 2011 | 6.31 | | 2010 | 6.28 | | 2009 | 6.62 | | 2008 | 6.57 | | 2007 | 6.54 | | 2006 | 6.74 | | 2005 | 6.78 | | 2004 | 6.74 | | 2003 | 6.75 | | 2002 | 7.67 | | 2001 | 7.74 | | 2000 | 7.7 | | 1999 | 7.3099999428 | | 1998 | 7.3099999428 | | 1997 | 7.3800001144 | | 1996 | 7.5599999428 | | 1995 | 7.6399998665 | | 1994 | 7.6944999695 | | 1993 | 7.8699002266 | | 1992 | 7.8621001244 | | 1991 | 8.9221000671 | | 1990 | 8.9198999405 | | 1989 | 8.9371004104614 | | 1988 | 8.9421997070312 | | 1987 | 8.9502000808716 | | 1986 | 8.9465999603272 | | 1985 | 8.9404001235962 | | 1984 | 8.8957996368408 | | 1982 | 8.8107004165649 | | 1981 | 8.7750997543335 | | 1980 | 8.7762002944946 | | 1960 | 7.2732706069946 |
|