| Year | Value | | 2004 | 4.2168406661192 | | 2003 | 3.3068131878688 | | 2002 | 3.578992891915 | | 2001 | 4.1036429089939 | | 2000 | 3.7327777207166 | | 1999 | 5.0130550316552 | | 1998 | 5.6642725044084 | | 1997 | 5.097809027026 | | 1996 | 5.0429327046564 | | 1995 | 4.7967629715724 | | 1994 | 5.1734018738224 | | 1993 | 5.3495627269473 | | 1992 | 5.3907299596955 | | 1991 | 5.0411937546811 | | 1990 | 5.1873742454728 | | 1989 | 5.1765677703586 | | 1988 | 4.8775231664392 | | 1987 | 5.3159797174204 | | 1986 | 6.4302518079638 | | 1985 | 5.8917777292258 | | 1984 | 5.7133129043241 | | 1983 | 5.4044431785816 | | 1982 | 4.1362707907791 | | 1981 | 3.7579568985352 | | 1980 | 3.4452983334773 |
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