| Year | Value | | 2019 | 2.6 | | 2018 | 2.43 | | 2017 | 2.61 | | 2016 | 2.6 | | 2015 | 2.53 | | 2014 | 2.69 | | 2013 | 3.07 | | 2011 | 3.13 | | 2010 | 3.5 | | 2009 | 3.49 | | 2008 | 3.57 | | 2007 | 3.69 | | 2006 | 3.79 | | 2005 | 3.86 | | 2004 | 3.97 | | 2003 | 4.13 | | 2002 | 4.29 | | 2001 | 4.22 | | 2000 | 4.29 | | 1999 | 4.3000001907 | | 1998 | 4.5 | | 1997 | 4.5999999046 | | 1996 | 4.6999998093 | | 1995 | 4.9000000954 | | 1994 | 5 | | 1993 | 5 | | 1992 | 5.0999999046 | | 1991 | 5.4000000954 | | 1990 | 5.5999999046 | | 1989 | 5.8000001907349 | | 1988 | 6 | | 1987 | 6.1999998092651 | | 1986 | 6.9000000953674 | | 1985 | 7 | | 1984 | 7.0999999046326 | | 1983 | 7.4000000953674 | | 1982 | 7.6999998092651 | | 1981 | 7.8000001907349 | | 1980 | 8.1000003814697 | | 1979 | 8.1999998092651 | | 1978 | 8.5 | | 1977 | 8.3999996185303 | | 1976 | 8.6000003814697 | | 1975 | 8.5 | | 1974 | 8.5 | | 1973 | 8.3000001907349 | | 1972 | 8.3999996185303 | | 1970 | 8.1000003814697 |
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