| Year | Value | | 1998 | 12,000 | | 1997 | 12,000 | | 1996 | 9,400 | | 1995 | 9,400 | | 1994 | 8,900 | | 1993 | 8,900 | | 1992 | 8,900 | | 1991 | 8,300 | | 1990 | 8,300 | | 1989 | 7,800 | | 1988 | 11,200 | | 1987 | 21,000 | | 1986 | 5,500 | | 1985 | 8,600 | | 1984 | 8,800 | | 1983 | 6,000 | | 1982 | 5,500 | | 1981 | 5,000 | | 1980 | 11,000 | | 1979 | 5,500 | | 1978 | 5,000 | | 1977 | 4,500 | | 1976 | 4,000 |
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