| Year | Value | | 2007 | 0.071508252052287 | | 2006 | 0.093056549749463 | | 2005 | 0.097454013887197 | | 2004 | 0.10636883393166 | | 2003 | 0.08029093657016 | | 2002 | 3.3341260404281 | | 2001 | 2.5952441830734 | | 2000 | 2.4668779972365 | | 1999 | 2.3996612242978 | | 1998 | 2.2353584024639 | | 1997 | 1.9680196801968 | | 1996 | 2.0986796386379 | | 1995 | 1.9820646081728 | | 1994 | 2.1124124284721 | | 1993 | 2.8146940822467 | | 1992 | 3.1956124314442 | | 1991 | 3.6313317678631 | | 1990 | 3.4815681685196 | | 1989 | 4.0029112081514 | | 1988 | 5.0538635456843 | | 1987 | 5.9258015779755 | | 1986 | 6.1059907834101 | | 1985 | 6.109375 | | 1984 | 7.0818070818071 | | 1983 | 7.2268907563025 | | 1982 | 7.5917859365277 | | 1981 | 7.9049198452184 | | 1980 | 9.9408553230209 | | 1979 | 11.61899144795 | | 1978 | 11.635865845311 | | 1977 | 11.444241316271 | | 1976 | 11.375 | | 1975 | 10.461538461539 | | 1974 | 11.536471805484 | | 1973 | 15.484710474951 | | 1972 | 16.225165562914 |
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