| Year | Value | | 2001 | 716 | | 2000 | 1,518 | | 1999 | 1,500 | | 1998 | 1,500 | | 1997 | 1,500 | | 1996 | 1,500 | | 1995 | 1,300 | | 1994 | 1,300 | | 1993 | 1,300 | | 1992 | 1,100 | | 1991 | 4,500 | | 1990 | 3,800 | | 1989 | 4,400 | | 1988 | 4,400 | | 1987 | 4,300 | | 1986 | 4,300 | | 1985 | 4,300 | | 1984 | 4,200 | | 1983 | 4,100 | | 1982 | 4,200 | | 1981 | 4,000 | | 1980 | 4,300 | | 1979 | 4,200 | | 1978 | 4,100 | | 1977 | 3,800 | | 1976 | 3,900 | | 1975 | 3,700 | | 1974 | 3,400 | | 1973 | 3,400 | | 1972 | 3,300 | | 1971 | 3,600 | | 1970 | 3,600 |
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