| Year | Value | | 1990 | 0.069993040017931 | | 1989 | 0.068217847494351 | | 1988 | 0.067359278542504 | | 1987 | 0.065932425483919 | | 1986 | 0.07713933077345 | | 1985 | 0.08196518948278 | | 1984 | 0.092352222366131 | | 1983 | 0.088837589323927 | | 1982 | 0.089434291453161 | | 1981 | 0.088167015829619 | | 1980 | 0.10056755078661 | | 1979 | 0.10860429317493 | | 1972 | 3.499743764947 |
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