| Year | Value | | 1994 | 15.092129101315 | | 1993 | 12.52460798252 | | 1992 | 9.8404391831084 | | 1991 | 9.5411797238 | | 1990 | 10.879640515891 | | 1989 | 11.956715971995 | | 1988 | 10.567792827513 | | 1987 | 8.3345711663965 | | 1986 | 8.6309216427661 | | 1985 | 11.470330261966 | | 1984 | 7.0632543822668 | | 1983 | 7.3829075312053 | | 1982 | 0.97086207669704 | | 1981 | 8.1378270601825 | | 1980 | 1.6057271662374 | | 1979 | 7.5188159910598 | | 1978 | 6.9229115606155 | | 1977 | 7.8292757521775 |
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