| Year | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | Annual |
| 1931 | 58.2 | 62.4 | 61.3 | 71.8 | 77.0 | 89.8 | 94.0 | 89.0 | 92.1 | 83.2 | 71.4 | 60.1 | 75.9 |
| 1932 | 60.9 | 68.8 | 64.1 | 77.0 | 80.7 | 87.8 | 92.9 | 90.2 | 83.8 | 74.7 | 59.8 | 51.8 | 74.4 |
| 1933 | 60.3 | 54.6 | 65.9 | 71.9 | 80.7 | 88.0 | 88.3 | 90.6 | 89.5 | 78.1 | 68.8 | 67.6 | 75.5 |
| 1934 | 56.2 | 62.1 | 67.0 | 78.4 | 81.9 | 91.8 | 95.5 | 95.7 | 87.1 | 83.9 | 70.2 | 55.9 | 77.2 |
| 1935 | 59.3 | 58.5 | 73.9 | 72.4 | 79.1 | 86.1 | 92.1 | 94.9 | 84.9 | 82.0 | 66.2 | 55.3 | 75.5 |
| 1936 | 59.4 | 57.6 | 73.6 | 75.4 | 80.9 | 91.8 | 89.6 | 94.2 | 88.5 | 74.8 | 64.4 | 60.5 | 75.9 |
| 1937 | 59.2 | 61.3 | 62.9 | 72.4 | 83.4 | 89.7 | 91.6 | 93.5 | 86.0 | 75.4 | 61.0 | 53.6 | 74.2 |
| 1938 | 55.1 | 66.3 | 73.7 | 72.3 | 81.0 | 88.4 | 90.6 | 91.1 | 89.7 | 84.1 | 66.9 | 59.5 | 76.6 |
| 1939 | 61.4 | 61.7 | 71.0 | 74.7 | 82.3 | 88.5 | 95.4 | 95.2 | 93.7 | 81.6 | 63.6 | 64.1 | 77.9 |
| 1940 | 45.4 | 58.0 | 69.8 | 73.5 | 79.5 | 83.5 | 89.3 | 87.4 | 83.3 | 80.6 | 64.5 | 62.4 | 73.2 |
| 1941 | 60.6 | 55.1 | 61.5 | 75.5 | 81.2 | 86.1 | 89.5 | 91.0 | 86.6 | 79.4 | 64.6 | 59.9 | 74.3 |
| 1942 | 56.0 | 56.3 | 67.5 | 74.7 | 79.8 | 87.1 | 89.8 | 90.8 | 83.3 | 79.1 | 71.1 | 60.6 | 74.8 |
| 1943 | 57.9 | 65.8 | 65.7 | 78.4 | 85.4 | 92.0 | 93.3 | 94.8 | 84.2 | 77.2 | 65.9 | 55.5 | 76.4 |
| 1944 | 55.8 | 64.8 | 67.9 | 74.7 | 80.0 | 89.8 | 93.8 | 94.6 | 86.6 | 82.0 | 66.1 | 54.5 | 75.9 |
| 1945 | 55.3 | 66.8 | 76.4 | 78.2 | 83.0 | 90.3 | 90.7 | 92.8 | 90.7 | 77.8 | 74.3 | 58.5 | 77.9 |
| 1946 | 56.2 | 64.5 | 73.8 | 81.2 | 82.1 | 87.2 | 91.9 | 92.2 | 86.4 | 80.3 | 70.0 | 65.6 | 77.7 |
| 1947 | 56.3 | 58.0 | 64.2 | 78.2 | 83.1 | 89.8 | 93.7 | 97.6 | 93.9 | 89.4 | 67.0 | 62.7 | 77.9 |
| 1948 | 52.9 | 61.7 | 71.9 | 81.5 | 84.3 | 92.3 | 94.5 | 97.7 | 89.6 | 81.2 | 67.5 | 64.4 | 78.3 |
| 1949 | 56.5 | 64.1 | 69.0 | 72.9 | 86.5 | 90.5 | 92.1 | 91.1 | 87.9 | 76.5 | 72.3 | 62.0 | 76.9 |
| 1950 | 64.7 | 65.2 | 68.2 | 73.4 | 84.8 | 87.5 | 89.6 | 92.9 | 85.6 | 81.7 | 68.0 | 59.6 | 76.7 |
| 1951 | 60.7 | 61.7 | 70.4 | 75.6 | 84.4 | 90.3 | 96.0 | 99.4 | 89.5 | 81.4 | 65.6 | 63.7 | 78.3 |
| 1952 | 67.6 | 64.8 | 67.7 | 73.8 | 82.3 | 92.1 | 92.7 | 95.9 | 90.6 | 78.6 | 67.3 | 58.2 | 77.7 |
| 1953 | 63.8 | 61.3 | 74.6 | 75.5 | 83.6 | 94.8 | 90.1 | 90.8 | 90.5 | 83.7 | 66.8 | 56.9 | 77.8 |
| 1954 | 61.0 | 71.5 | 68.2 | 79.2 | 78.8 | 91.2 | 97.9 | 99.4 | 97.2 | 84.4 | 70.8 | 64.5 | 80.4 |
| 1955 | 57.6 | 62.5 | 73.6 | 81.3 | 87.4 | 90.9 | 95.2 | 93.0 | 93.8 | 84.0 | 70.1 | 62.5 | 79.4 |
| 1956 | 63.1 | 65.7 | 71.7 | 77.7 | 88.0 | 91.5 | 98.6 | 99.7 | 95.3 | 88.2 | 70.1 | 67.7 | 81.5 |
| 1957 | 60.2 | 69.8 | 69.5 | 77.0 | 85.4 | 89.3 | 96.7 | 95.8 | 88.0 | 77.2 | 66.6 | 67.0 | 78.6 |
| 1958 | 59.1 | 56.6 | 67.5 | 77.7 | 86.7 | 92.1 | 94.5 | 95.8 | 86.5 | 79.3 | 71.6 | 60.2 | 77.4 |
| 1959 | 56.5 | 63.9 | 72.1 | 75.3 | 87.5 | 91.6 | 94.5 | 94.4 | 90.5 | 81.8 | 65.3 | 63.0 | 78.1 |
| 1960 | 57.1 | 58.1 | 62.3 | 80.3 | 85.7 | 92.3 | 95.3 | 93.5 | 90.9 | 83.3 | 70.2 | 55.2 | 77.0 |