| Year | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | Annual |
| 1991 | 52.2 | 64.0 | 71.8 | 77.2 | 83.0 | 90.6 | 94.4 | 92.7 | 86.0 | 83.3 | 63.2 | 61.0 | 76.7 |
| 1992 | 56.5 | 64.6 | 71.2 | 76.1 | 79.9 | 87.3 | 92.1 | 89.5 | 86.4 | 81.3 | 63.1 | 57.8 | 75.5 |
| 1993 | 56.2 | 62.0 | 66.6 | 73.8 | 81.2 | 89.9 | 95.7 | 98.7 | 92.0 | 77.5 | 63.8 | 60.2 | 76.4 |
| 1994 | 57.0 | 61.9 | 70.5 | 79.4 | 81.6 | 92.9 | 94.7 | 92.1 | 88.8 | 78.7 | 72.7 | 61.2 | 77.6 |
| 1995 | 59.7 | 66.0 | 69.1 | 76.9 | 85.0 | 91.1 | 95.5 | 97.2 | 90.6 | 82.3 | 70.4 | 61.5 | 78.8 |
| 1996 | 62.4 | 66.3 | 67.2 | 78.7 | 89.0 | 91.5 | 96.6 | 92.9 | 87.6 | 79.4 | 69.0 | 65.7 | 78.9 |
| 1997 | 58.7 | 62.5 | 73.7 | 72.8 | 82.5 | 89.7 | 95.8 | 92.5 | 91.0 | 78.4 | 63.6 | 57.4 | 76.6 |
| 1998 | 61.8 | 63.9 | 68.4 | 76.4 | 89.9 | 97.9 | 103.0 | 98.3 | 92.9 | 80.0 | 69.9 | 61.8 | 80.4 |
| 1999 | 65.5 | 70.0 | 70.3 | 81.9 | 84.8 | 89.9 | 94.3 | 101.5 | 92.1 | 82.3 | 76.3 | 65.6 | 81.2 |
| 2000 | 63.9 | 72.6 | 75.8 | 77.5 | 87.0 | 90.4 | 98.2 | 102.0 | 94.6 | 82.0 | 64.7 | 53.0 | 80.1 |
| 2001 | 54.1 | 66.1 | 62.6 | 80.4 | 87.4 | 89.0 | 95.5 | 96.3 | 87.0 | 78.8 | 74.6 | 65.3 | 78.1 |
| 2002 | 64.0 | 61.8 | 69.0 | 80.7 | 87.2 | 91.0 | 94.5 | 95.5 | 91.9 | 77.5 | 67.0 | 61.0 | 78.4 |
| 2003 | 58.0 | 60.1 | 69.9 | 81.2 | 88.8 | 91.9 | 94.3 | 96.3 | 86.8 | 82.1 | 74.2 | 64.7 | 79.0 |
| 2004 | 60.8 | 56.5 | 74.0 | 77.5 | 85.2 | 88.2 | 93.4 | 93.8 | 90.5 | 83.9 | 67.6 | 60.7 | 77.7 |
| 2005 | 61.9 | 62.4 | 70.0 | 77.5 | 84.4 | 93.6 | 95.1 | 97.3 | 96.6 | 81.2 | 73.4 | 61.1 | 79.5 |
| 2006 | 67.6 | 60.3 | 72.0 | 83.3 | 85.9 | 92.1 | 94.2 | 97.7 | 89.2 | 79.2 | 69.8 | 60.6 | 79.3 |
| 2007 | 56.0 | 59.4 | 75.1 | 73.5 | 83.6 | 88.9 | 88.5 | 94.5 | 89.8 | 82.7 | 70.2 | 58.8 | 76.8 |
| 2008 | 58.1 | 66.0 | 72.3 | ||||||||||