| Year | Value | | 1999 | 17.799112604011 | | 1998 | 18.67935414781 | | 1997 | 16.672201077933 | | 1996 | 18.915097007749 | | 1995 | 18.541981618794 | | 1994 | 15.573456079785 | | 1993 | 20.336243129648 | | 1992 | 20.87815458569 | | 1991 | 24.988313023966 | | 1981 | 26.004938271605 | | 1980 | 29.707385044124 | | 1979 | 29.055007052186 | | 1978 | 23.092833678063 | | 1977 | 22.779283348389 | | 1976 | 20.718671821697 | | 1975 | 33.038147138965 | | 1974 | 31.381329658973 | | 1973 | 25.515873015873 |
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