| Year | Value | | 1985 | 9.8135964912281 | | 1984 | 9.6434049079755 | | 1983 | 10.128232992828 | | 1982 | 13.12625250501 | | 1981 | 14.795447554599 | | 1980 | 12.246489859594 | | 1979 | 10.325534079349 | | 1978 | 10.1243339254 | | 1977 | 15.966964900207 | | 1976 | 18.63453815261 | | 1975 | 18.29134720701 | | 1974 | 19.650067294751 | | 1973 | 19.793205317577 | | 1972 | 20.795660036166 |
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