| Year | Value | | 1991 | 16.2000007629 | | 1990 | 17.7999992371 | | 1989 | 18.1000003815 | | 1988 | 16 | | 1987 | 16.3999996185 | | 1986 | 16.6000003815 | | 1985 | 18.5 | | 1984 | 17.5 | | 1983 | 18.7999992371 | | 1982 | 21.1000003815 | | 1981 | 21.1000003815 | | 1980 | 18.1000003815 | | 1979 | 17.2000007629 | | 1978 | 14.6999998093 | | 1977 | 13.1000003815 | | 1976 | 13.1999998093 | | 1975 | 11.5 | | 1974 | 10.8000001907 | | 1973 | 8.3000001907 | | 1972 | 8 | | 1971 | 7.0999999046 | | 1970 | 5.8000001907 |
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