| Year | Value | | 1998 | 20,400 | | 1997 | 20,500 | | 1996 | 20,500 | | 1995 | 20,500 | | 1994 | 20,500 | | 1993 | 20,500 | | 1992 | 20,500 | | 1991 | 20,500 | | 1990 | 20,500 | | 1989 | 20,500 | | 1988 | 20,500 | | 1987 | 20,500 | | 1986 | 19,500 | | 1985 | 22,500 | | 1984 | 21,900 | | 1983 | 21,000 | | 1982 | 20,600 | | 1981 | 22,200 | | 1980 | 22,200 | | 1979 | 20,000 | | 1978 | 20,000 |
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