| Year | Value | | 2012 | 0.46767875690694 | | 2011 | 5.85295 | | 2010 | 5.6089557071307 | | 2009 | 9.804 | | 2008 | 11.755 | | 2007 | 27.161 | | 2006 | 20 | | 2005 | 21.429 | | 2004 | 23.996 | | 2003 | 28.37 | | 2002 | 25.151 | | 2001 | 32.294 | | 2000 | 50.852 | | 1999 | 39.9000015259 | | 1998 | 37 | | 1997 | 34.9000015259 | | 1996 | 29.2999992371 | | 1995 | 28.5 | | 1994 | 17.7000007629 | | 1993 | 10.5 | | 1992 | 6.5 | | 1991 | 5 | | 1990 | 6.3000001907 | | 1989 | 5.8000001907 | | 1988 | 5.8000001907 | | 1987 | 7.3000001907 | | 1986 | 8.8000001907 | | 1985 | 10.3000001907 | | 1984 | 8.6000003815 | | 1983 | 8.8000001907 | | 1982 | 9.1999998093 | | 1981 | 9.6999998093 | | 1980 | 9.1999998093 | | 1979 | 7.8000001907 | | 1978 | 5.9000000954 | | 1977 | 5.5 | | 1976 | 5.1999998093 | | 1975 | 5.5999999046 | | 1974 | 6 | | 1973 | 5.8000001907 | | 1972 | 5.9000000954 | | 1971 | 7.5999999046 | | 1970 | 6.1999998093 |
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