| Year | Value | | 2017 | 2.87 | | 2016 | 2.77 | | 2015 | 2.8 | | 2014 | 2.83 | | 2013 | 2.89 | | 2012 | 2.93 | | 2011 | 2.97 | | 2010 | 3.05 | | 2009 | 3.08 | | 2008 | 3.13 | | 2007 | 3.14 | | 2006 | 3.18 | | 2005 | 3.2 | | 2004 | 3.26 | | 2003 | 3.33 | | 2002 | 3.39 | | 2001 | 3.47 | | 2000 | 3.49 | | 1999 | 3.5999999046 | | 1998 | 3.7000000477 | | 1997 | 3.7999999523 | | 1996 | 3.9000000954 | | 1995 | 4.0999999046 | | 1994 | 4.3000001907 | | 1993 | 4.5 | | 1992 | 4.5999999046 | | 1991 | 4.8000001907 | | 1990 | 4.9000000954 | | 1989 | 5 | | 1988 | 5.0999999046326 | | 1987 | 5.1999998092651 | | 1986 | 5.4000000953674 | | 1985 | 5.5 | | 1984 | 5.6999998092651 | | 1983 | 5.8000001907349 | | 1982 | 5.9000000953674 | | 1981 | 5.9000000953674 | | 1980 | 6 | | 1979 | 6.0999999046326 | | 1978 | 6.1999998092651 | | 1977 | 6.4000000953674 | | 1976 | 6.5999999046326 | | 1975 | 6.8000001907349 | | 1974 | 7.0999999046326 | | 1973 | 7.1999998092651 | | 1972 | 7.4000000953674 | | 1971 | 7.5 | | 1970 | 7.9000000953674 | | 1965 | 8.8000001907349 | | 1960 | 9.1999998092651 |
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