| Year | Value | | 2019 | 2.52 | | 2018 | 2.55 | | 2017 | 2.53 | | 2016 | 2.6 | | 2015 | 2.62 | | 2014 | 2.67 | | 2013 | 2.72 | | 2012 | 2.79 | | 2011 | 2.8 | | 2010 | 2.78 | | 2009 | 2.8 | | 2008 | 2.85 | | 2007 | 2.96 | | 2006 | 3.02 | | 2005 | 3.1 | | 2004 | 3.44 | | 2003 | 3.5 | | 2002 | 3.64 | | 2001 | 3.69 | | 2000 | 3.77 | | 1999 | 3.9000000954 | | 1998 | 4.1999998093 | | 1997 | 4.4000000954 | | 1996 | 4.5999999046 | | 1995 | 5 | | 1993 | 5.4000000954 | | 1992 | 5.5 | | 1991 | 5.8000001907 | | 1990 | 6 | | 1989 | 6.3000001907349 | | 1988 | 6.5 | | 1987 | 6.5999999046326 | | 1986 | 6.6999998092651 | | 1985 | 6.8000001907349 | | 1984 | 6.8000001907349 | | 1983 | 6.8000001907349 | | 1982 | 6.8000001907349 | | 1981 | 6.9000000953674 | | 1980 | 6.8000001907349 | | 1979 | 6.8000001907349 | | 1978 | 6.9000000953674 | | 1977 | 6.9000000953674 | | 1976 | 6.9000000953674 | | 1970 | 7 | | 1960 | 6.1999998092651 |
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