| Year | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | Annual |
| 1991 | 43.6 | 51.6 | 59.3 | 67.3 | 74.5 | 81.0 | 84.0 | 82.4 | 75.3 | 69.0 | 51.0 | 51.7 | 66.0 |
| 1992 | 45.9 | 52.9 | 58.5 | 64.3 | 70.0 | 77.7 | 82.3 | 78.3 | 75.8 | 66.8 | 51.6 | 49.0 | 64.5 |
| 1993 | 46.8 | 50.7 | 54.4 | 61.4 | 70.8 | 80.4 | 84.6 | 86.1 | 78.7 | 66.3 | 51.3 | 48.8 | 65.0 |
| 1994 | 46.5 | 49.5 | 57.6 | 65.8 | 71.1 | 81.9 | 83.2 | 80.7 | 75.5 | 67.4 | 60.4 | 50.8 | 65.8 |
| 1995 | 48.5 | 51.6 | 58.5 | 63.9 | 74.5 | 78.2 | 83.5 | 85.1 | 77.1 | 66.4 | 56.3 | 51.8 | 66.3 |
| 1996 | 48.3 | 54.3 | 53.6 | 64.8 | 79.7 | 81.4 | 86.4 | 84.0 | 77.3 | 67.6 | 59.0 | 54.6 | 67.6 |
| 1997 | 49.4 | 53.8 | 64.7 | 64.0 | 73.8 | 79.2 | 84.4 | 83.0 | 79.1 | 67.5 | 53.7 | 46.8 | 66.6 |
| 1998 | 52.2 | 51.7 | 56.8 | 64.1 | 79.0 | 87.9 | 91.5 | 87.9 | 84.1 | 71.2 | 61.4 | 52.7 | 70.0 |
| 1999 | 54.1 | 58.9 | 60.1 | 72.0 | 75.8 | 82.6 | 85.8 | 87.9 | 77.7 | 68.0 | 61.6 | 52.5 | 69.7 |
| 2000 | 53.7 | 60.8 | 65.0 | 66.6 | 77.9 | 81.0 | 85.7 | 87.5 | 80.5 | 70.5 | 54.9 | 44.1 | 69.0 |
| 2001 | 44.6 | 55.0 | 52.6 | 69.1 | 75.5 | 78.6 | 84.4 | 84.8 | 76.1 | 65.3 | 62.5 | 53.6 | 66.9 |
| 2002 | 52.8 | 49.1 | 59.2 | 71.5 | 76.3 | 80.7 | 83.6 | 84.6 | 80.0 | 69.1 | 55.2 | 51.1 | 67.8 |
| 2003 | 47.6 | 53.4 | 59.1 | 69.3 | 78.5 | 81.5 | 83.6 | 84.9 | 76.6 | 70.0 | 63.9 | 52.1 | 68.4 |
| 2004 | 51.9 | 49.0 | 63.3 | 66.6 | 75.6 | 79.8 | 83.1 | 81.4 | 78.0 | 75.0 | 59.0 | 47.4 | 67.5 |
| 2005 | 52.0 | 53.4 | 56.9 | 64.2 | 72.3 | 82.1 | 84.3 | 85.2 | 83.1 | 66.8 | 60.5 | 46.5 | 67.3 |
| 2006 | 52.3 | 47.1 | 60.2 | 70.8 | 74.7 | 80.3 | 83.8 | 86.2 | 74.5 | 67.3 | 56.6 | 49.7 | 67.0 |
| 2007 | 45.9 | 48.2 | 62.4 | 62.1 | 73.8 | 80.0 | 80.1 | 84.7 | 79.0 | 70.6 | 58.6 | 48.4 | 66.2 |
| 2007 | 45.7 | 52.6 | 58.7 | 64.3 | 71.1 | 81.4 | |||||||