| Year | Value | | 2018 | 3.14 | | 2017 | 3.18 | | 2016 | 3.17 | | 2015 | 3.2 | | 2014 | 3.21 | | 2013 | 3.31 | | 2012 | 3.42 | | 2011 | 3.52 | | 2010 | 3.64 | | 2009 | 3.69 | | 2008 | 3.79 | | 2007 | 3.91 | | 2006 | 3.99 | | 2005 | 4.04 | | 2004 | 4.02 | | 2003 | 4.18 | | 2002 | 4.44 | | 2001 | 4.61 | | 2000 | 4.71 | | 1999 | 4.9000000954 | | 1998 | 5.5 | | 1997 | 5.9000000954 | | 1996 | 6.5 | | 1995 | 6.3000001907 | | 1994 | 6.5999999046 | | 1993 | 6.6999998093 | | 1992 | 6.9000000954 | | 1991 | 6.8000001907 | | 1990 | 7.1999998093 | | 1989 | 7.0999999046326 | | 1988 | 7.5 | | 1987 | 7.8000001907349 | | 1986 | 8 | | 1985 | 8.3000001907349 | | 1984 | 8.5 | | 1983 | 8.8999996185303 | | 1982 | 9.1000003814697 | | 1981 | 9.3999996185303 | | 1980 | 9.6000003814697 | | 1979 | 9.8000001907349 | | 1978 | 10 | | 1977 | 10.300000190735 | | 1976 | 10.39999961853 | | 1975 | 10.60000038147 | | 1974 | 10.60000038147 | | 1973 | 10.5 | | 1972 | 10.60000038147 | | 1971 | 10.60000038147 | | 1970 | 10.60000038147 | | 1969 | 10.300000190735 | | 1968 | 10.10000038147 | | 1967 | 9.8999996185303 | | 1966 | 9.6999998092651 | | 1965 | 9.6000003814697 | | 1964 | 9.5 | | 1963 | 9.3000001907349 | | 1962 | 9.1999998092651 | | 1961 | 9 | | 1960 | 8.8999996185303 |
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