1971  13.2  
1972   9.3
1973  14.9
1974  13.9
1975  14.8
1976  13.3
1977  15.0
1978  15.3  
1979  15.1
1980  13.1
1981  17.0
1982  19.3
1983  15.6
1984  15.3