| Year | Value | | 2018 | 4.57 | | 2017 | 4.69 | | 2016 | 4.76 | | 2015 | 4.96 | | 2014 | 5.01 | | 2013 | 5.01 | | 2012 | 5.53 | | 2011 | 5.36 | | 2010 | 5.27 | | 2009 | 5.37 | | 2008 | 5.63 | | 2007 | 5.48 | | 2006 | 5.55 | | 2005 | 5.38 | | 2004 | 5.64 | | 2003 | 5.73 | | 2002 | 5.98 | | 2001 | 6.6 | | 2000 | 7.04 | | 1999 | 7.5300002098 | | 1998 | 7.5799999237 | | 1997 | 7.7100000381 | | 1996 | 7.9000000954 | | 1995 | 8.3500003815 | | 1994 | 8.5613002777 | | 1993 | 9.622300148 | | 1992 | 9.6816997528 | | 1991 | 11.2891998291 | | 1990 | 11.6106004715 | | 1989 | 11.930800437927 | | 1988 | 12.026700019836 | | 1987 | 12.136500358582 | | 1986 | 12.265199661255 | | 1985 | 12.274399757385 | | 1980 | 12.185099601746 |
|