| Year | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | Annual |
| 1991 | 70.0 | 76.0 | 89.0 | 88.0 | 92.0 | 95.0 | 99.0 | 100.0 | 95.0 | 93.0 | 80.0 | 78.0 | 100.0 |
| 1992 | 68.0 | 76.0 | 84.0 | 86.0 | 88.0 | 94.0 | 96.0 | 98.0 | 93.0 | 89.0 | 85.0 | 70.0 | 98.0 |
| 1993 | 73.0 | 80.0 | 85.0 | 85.0 | 88.0 | 96.0 | 102.0 | 104.0 | 100.0 | 93.0 | 83.0 | 76.0 | 104.0 |
| 1994 | 75.0 | 81.0 | 83.0 | 89.0 | 90.0 | 98.0 | 100.0 | 96.0 | 96.0 | 94.0 | 86.0 | 82.0 | 100.0 |
| 1995 | 82.0 | 82.0 | 90.0 | 87.0 | 95.0 | 97.0 | 104.0 | 104.0 | 103.0 | 94.0 | 82.0 | 84.0 | 104.0 |
| 1996 | 78.0 | 92.0 | 86.0 | 91.0 | 100.0 | 98.0 | 102.0 | 102.0 | 94.0 | 89.0 | 84.0 | 82.0 | 102.0 |
| 1997 | 84.0 | 85.0 | 89.0 | 86.0 | 90.0 | 95.0 | 101.0 | 98.0 | 100.0 | 93.0 | 86.0 | 74.0 | 101.0 |
| 1998 | 70.0 | 71.0 | 86.0 | 87.0 | 100.0 | 108.0 | 106.0 | 108.0 | 104.0 | 98.0 | 84.0 | 82.0 | 108.0 |
| 1999 | 82.0 | 84.0 | 82.0 | 91.0 | 90.0 | 94.0 | 100.0 | 108.0 | 102.0 | 92.0 | 85.0 | 80.0 | 108.0 |
| 2000 | 83.0 | 86.0 | 89.0 | 89.0 | 94.0 | 96.0 | 105.0 | 108.0 | 112.0 | 95.0 | 87.0 | 74.0 | 112.0 |
| 2001 | 75.0 | 80.0 | 80.0 | 88.0 | 93.0 | 95.0 | 102.0 | 102.0 | 95.0 | 90.0 | 85.0 | 82.0 | 102.0 |
| 2002 | 84.0 | 77.0 | 85.0 | 94.0 | 95.0 | 95.0 | 100.0 | 100.0 | 99.0 | 98.0 | 88.0 | 77.0 | 100.0 |
| 2003 | 82.0 | 77.0 | 83.0 | 90.0 | 100.0 | 100.0 | 98.0 | 103.0 | 93.0 | 96.0 | 88.0 | 75.0 | 103.0 |
| 2004 | 80.0 | 75.0 | 85.0 | 87.0 | 92.0 | 95.0 | 99.0 | 103.0 | 100.0 | 94.0 | 86.0 | 78.0 | 103.0 |
| 2005 | 80.0 | 83.0 | 88.0 | 89.0 | 98.0 | 99.0 | 101.0 | 103.0 | 104.0 | 92.0 | 87.0 | 81.0 | 104.0 |
| 2006 | 84.0 | 75.0 | 87.0 | 93.0 | 94.0 | 98.0 | 104.0 | 103.0 | 96.0 | 93.0 | 69.8 | 76.0 | 104.0 |
| 2007 | 74.0 | 82.0 | 84.0 | 88.0 | 90.0 | 94.0 | 94.0 | 101.0 | 94.0 | 92.0 | 84.0 | 74.0 | 101.0 |
| 2008 | 76.0 | 78.0 | 86.0 | ||||||||||