| Year | Value | | 1989 | 10.70406280114 | | 1988 | 9.320710132217 | | 1987 | 8.7450437872436 | | 1986 | 8.7071753370947 | | 1985 | 10.10003835933 | | 1984 | 8.3155116771755 | | 1983 | 7.0495916600809 | | 1982 | 6.9826687155346 | | 1981 | 7.4053177574076 | | 1980 | 8.1151506040211 | | 1979 | 7.2191073930593 | | 1978 | 5.4857408403015 | | 1977 | 6.223399336276 | | 1976 | 6.1092948452258 | | 1975 | 8.0328975542952 | | 1974 | 10.503159469959 | | 1973 | 9.434464780036 | | 1972 | 9.3759190421334 |
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