| Year | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | Annual |
| 1961 | 55.1 | 66.8 | 76.0 | 76.2 | 85.8 | 87.2 | 91.2 | 94.5 | 90.1 | 80.5 | 66.3 | 62.1 | 77.7 |
| 1962 | 56.7 | 72.4 | 67.5 | 77.6 | 87.5 | 91.6 | 95.8 | 98.2 | 90.2 | 83.9 | 69.3 | 59.6 | 79.2 |
| 1963 | 51.5 | 62.9 | 76.1 | 82.9 | 88.4 | 94.1 | 95.5 | 98.0 | 90.0 | 88.9 | 71.5 | 53.1 | 79.5 |
| 1964 | 59.3 | 57.6 | 71.1 | 81.2 | 86.5 | 91.3 | 97.3 | 96.5 | 87.9 | 79.7 | 72.4 | 63.0 | 78.7 |
| 1965 | 64.6 | 57.5 | 60.9 | 82.9 | 84.4 | 91.2 | 95.9 | 96.4 | 90.1 | 78.8 | 75.2 | 65.1 | 78.7 |
| 1966 | 53.4 | 60.1 | 69.5 | 78.6 | 83.1 | 89.9 | 96.5 | 91.6 | 87.9 | 79.3 | 73.4 | 59.7 | 77.0 |
| 1967 | 59.7 | 60.4 | 76.6 | 83.4 | 84.1 | 92.0 | 91.4 | 94.5 | 87.3 | 81.8 | 71.6 | 62.5 | 78.9 |
| 1968 | 53.8 | 56.8 | 66.1 | 77.1 | 84.4 | 88.2 | 91.1 | 93.9 | 86.4 | 80.8 | 66.4 | 60.9 | 75.5 |
| 1969 | 59.4 | 60.7 | 60.8 | 77.8 | 83.9 | 91.4 | 98.0 | 96.0 | 91.1 | 81.5 | 69.9 | 62.4 | 77.8 |
| 1970 | 52.5 | 616 | 65.9 | 79.2 | 84.4 | 90.7 | 93.8 | 96.3 | 90.7 | 75.9 | 66.7 | 68.5 | 77.3 |
| 1971 | 62.9 | 64.0 | 69.9 | 78.0 | 83.0 | 93.2 | 95.0 | 92.0 | 88.3 | 82.8 | 70.9 | 65.2 | 78.8 |
| 1972 | 60.9 | 63.5 | 74.5 | 79.8 | 85.0 | 91.9 | 90.5 | 93.5 | 91.9 | 77.5 | 61.6 | 56.3 | 77.3 |
| 1973 | 53.2 | 59.4 | 72.4 | 72.7 | 84.9 | 86.9 | 92.1 | 91.0 | 86.7 | 80.5 | 75.8 | 62.1 | 76.6 |
| 1974 | 57.0 | 64.6 | 73.7 | 76.6 | 84.2 | 87.4 | 93.1 | 91.0 | 79.4 | 78.5 | 65.9 | 59.0 | 75.9 |
| 1975 | 62.0 | 61.7 | 67.1 | 74.2 | 82.8 | 88.0 | 90.5 | 91.2 | 84.7 | 79.8 | 68.5 | 59.4 | 75.9 |
| 1976 | 59.9 | 69.5 | 69.3 | 77.2 | 78.3 | 86.0 | 89.2 | 92.2 | 86.3 | 71.9 | 60.6 | 58.1 | 74.9 |
| 1977 | 48.8 | 63.4 | 70.1 | 76.5 | 84.9 | 91.1 | 95.1 | 92.1 | 88.2 | 79.1 | 67.9 | 60.2 | 76.5 |
| 1978 | 44.6 | 49.1 | 66.7 | 77.8 | 84.5 | 91.3 | 97.5 | 94.4 | 86.1 | 70.9 | 60.1 | 75.0 | |
| 1979 | 46.0 | 53.8 | 74.3 | 87.6 | 90.2 | 90.6 | 83.9 | 82.3 | 65.2 | 61.0 | 73.6 | ||
| 1980 | 57.7 | 58.5 | 66.6 | 74.4 | 82.1 | 92.3 | 98.3 | 96.0 | 92.7 | 77.5 | 65.6 | 61.5 | 77.0 |
| 1981 | 56.7 | 60.9 | 68.2 | 81.1 | 80.4 | 89.5 | 92.4 | 93.7 | 85.7 | 76.0 | 70.8 | 58.9 | 76.2 |
| 1982 | 58.6 | 56.2 | 69.5 | 72.7 | 82.7 | 89.6 | 92.0 | 95.3 | 89.6 | 76.2 | 65.9 | 61.4 | 75.9 |
| 1983 | 53.9 | 58.1 | 67.1 | 70.9 | 81.5 | 85.0 | 91.0 | 92.0 | 87.1 | 79.5 | 68.4 | 49.2 | 73.7 |
| 1984 | 53.1 | 64.5 | 68.7 | 78.0 | 84.9 | 89.5 | 94.4 | 95.3 | 86.6 | 78.3 | 66.9 | 67.6 | 77.4 |
| 1985 | 50.5 | 56.5 | 72.2 | 79.1 | 84.1 | 91.2 | 92.2 | 95.8 | 90.4 | 77.5 | 70.7 | 56.1 | 76.4 |
| 1986 | 61.4 | 66.0 | 73.1 | 77.4 | 81.4 | 88.5 | 93.2 | 91.9 | 87.7 | 76.4 | 65.2 | 55.2 | 76.5 |
| 1987 | 56.7 | 60.5 | 68.4 | 79.1 | 85.0 | 88.5 | 91.9 | 96.2 | 86.4 | 77.7 | 69.4 | 60.9 | 76.8 |
| 1988 | 52.5 | 60.0 | 69.3 | 78.8 | 84.3 | 92.1 | 92.5 | 93.7 | 89.2 | 78.0 | 70.6 | 61.1 | 76.9 |
| 1989 | 60.4 | 55.8 | 68.8 | 77.5 | 85.9 | 84.4 | 89.0 | 91.1 | 86.3 | 80.9 | 71.7 | 53.6 | 75.5 |
| 1990 | 63.5 | 68.3 | 70.3 | 77.3 | 83.6 | 93.4 | 94.0 | 96.3 | 91.5 | 80.1 | 73.5 | 60.4 | 79.4 |