| Year | Value | | 1987 | 0.021248565721814 | | 1986 | 0.026573364630852 | | 1985 | 0.027285129604366 | | 1984 | 0.038040893961008 | | 1983 | 0.046906759785271 | | 1982 | 0.054179985913204 | | 1981 | 0.043972956631672 | | 1980 | 0.045039972976016 | | 1979 | 0.053676865271068 | | 1978 | 0.057666431729352 | | 1977 | 0.068357372342607 | | 1976 | 0.075131480090158 | | 1975 | 0.065979381443299 | | 1974 | 0.47220611382653 | | 1973 | 0.8687424737657 | | 1972 | 1.6393442622951 |
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