| Year | Value | | 2018 | 16.199317 | | 2017 | 15.587857428 | | 2016 | 0.9519664068 | | 2015 | 1.218202664 | | 2014 | 10.619112 | | 2013 | 10.619112 | | 2012 | 15.335628318448 | | 2011 | 14.521373189766 | | 2010 | 11.718644551 | | 1990 | 42.7999992371 | | 1989 | 72.9000015259 | | 1988 | 70.4000015259 | | 1987 | 59.5 | | 1986 | 59.2000007629 | | 1985 | 54.5999984741 | | 1984 | 52 | | 1983 | 44.2999992371 | | 1982 | 53.7000007629 | | 1981 | 49.2000007629 | | 1980 | 52 | | 1979 | 46.5999984741 | | 1978 | 38.2000007629 | | 1977 | 30 | | 1976 | 13.8000001907 | | 1975 | 6.3000001907 | | 1974 | 4.9000000954 | | 1973 | 2.2999999523 | | 1972 | 2.2999999523 | | 1971 | 2 | | 1970 | 1.7999999523 |
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