| Year | Value | | 2018 | 2.97 | | 2017 | 2.97 | | 2016 | 2.97 | | 2015 | 2.98 | | 2014 | 2.97 | | 2013 | 2.96 | | 2012 | 2.99 | | 2011 | 3.05 | | 2010 | 3.11 | | 2009 | 3.16 | | 2008 | 3.2 | | 2007 | 3.25 | | 2006 | 3.29 | | 2005 | 3.34 | | 2004 | 3.4 | | 2003 | 3.44 | | 2002 | 3.53 | | 2001 | 3.58 | | 2000 | 3.65 | | 1999 | 4.0999999046 | | 1998 | 4.1999998093 | | 1997 | 4.1999998093 | | 1996 | 4.3000001907 | | 1995 | 4.3000001907 | | 1994 | 4.4000000954 | | 1993 | 4.4000000954 | | 1992 | 4.5 | | 1991 | 4.5999999046 | | 1990 | 4.5999999046 | | 1989 | 4.6999998092651 | | 1988 | 4.6999998092651 | | 1987 | 4.8000001907349 | | 1986 | 4.8000001907349 | | 1985 | 4.9000000953674 | | 1984 | 5 | | 1983 | 5.1999998092651 | | 1982 | 5.4000000953674 | | 1981 | 5.4000000953674 | | 1980 | 5.4000000953674 | | 1979 | 5.5 | | 1970 | 4.6999998092651 |
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