| Year | Value | | 1994 | 0.013039266820654 | | 1993 | 0.1025641025641 | | 1992 | 0.99627965405614 | | 1991 | 1.9573442685324 | | 1990 | 2.9060903366968 | | 1989 | 3.5916478967553 | | 1988 | 4.4363086591765 | | 1987 | 4.583591786099 | | 1986 | 5.0483372383784 | | 1985 | 9.3897805691246 | | 1984 | 9.0651558073655 | | 1983 | 9.4540714614323 | | 1982 | 10.493205837947 | | 1981 | 8.4168229306188 | | 1980 | 8.607014499938 | | 1979 | 19.836312728839 | | 1978 | 19.389587073609 | | 1977 | 25.63100290373 | | 1976 | 22.745701104432 | | 1975 | 19.712454529707 | | 1974 | 20.353063343718 | | 1973 | 20.088972137673 | | 1972 | 19.941262848752 |
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