| Year | Value | | 2018 | 4.43 | | 2017 | 4.5 | | 2016 | 4.49 | | 2015 | 4.51 | | 2014 | 4.54 | | 2013 | 4.55 | | 2012 | 4.54 | | 2011 | 4.62 | | 2010 | 4.57 | | 2009 | 4.6 | | 2008 | 4.74 | | 2007 | 4.66 | | 2006 | 4.77 | | 2005 | 4.83 | | 2004 | 4.8 | | 2003 | 4.96 | | 2002 | 5.09 | | 2001 | 5.16 | | 2000 | 5.4 | | 1999 | 5.5500001907 | | 1998 | 5.6199998856 | | 1997 | 5.6700000763 | | 1996 | 5.6599998474 | | 1995 | 5.75 | | 1994 | 5.7768998146 | | 1993 | 5.8007001877 | | 1992 | 5.9369997978 | | 1991 | 6.011300087 | | 1990 | 6.0408000946 | | 1989 | 6.0633997917175 | | 1988 | 6.1380000114441 | | 1987 | 6.2153000831604 | | 1986 | 6.2678999900818 | | 1985 | 6.3260998725891 | | 1984 | 6.5009999275208 | | 1982 | 6.6795001029968 | | 1980 | 6.9526000022888 |
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