| Year | Value | | 2009 | 3.758 | | 2008 | 4.39 | | 2007 | 4.229 | | 2006 | 4.146 | | 2005 | 4.274 | | 2004 | 4.5 | | 2003 | 23.175 | | 2002 | 11.72 | | 2001 | 13.318 | | 2000 | 14.476 | | 1999 | 13.3000001907 | | 1998 | 12.3999996185 | | 1997 | 8.3000001907 | | 1996 | 4.3000001907 | | 1995 | 4.1999998093 | | 1994 | 3.7999999523 | | 1993 | 3.5999999046 | | 1992 | 2.7999999523 | | 1991 | 2.5999999046 | | 1990 | 2.5999999046 | | 1989 | 9.3999996185 | | 1988 | 2.4000000954 | | 1987 | 2.2000000477 | | 1986 | 2.5999999046 | | 1985 | 1.8999999762 | | 1984 | 2 | | 1983 | 1.5 | | 1982 | 1 | | 1981 | 1.1000000238 | | 1980 | 0.6999999881 | | 1979 | 0.6999999881 | | 1978 | 0.200000003 | | 1977 | 0.3000000119 | | 1976 | 0.200000003 | | 1975 | 0.1000000015 | | 1974 | 0.1000000015 | | 1973 | 0.200000003 | | 1972 | 0.1000000015 | | 1971 | 0.5 | | 1970 | 0.3000000119 |
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