| Year | Range (%) | Mode (%) | Study figures |
| Pre 2000 | 55–73 | 70 | 55, 70, 73 |
| 2000 | 66–70 | 67.5 | 66.1, 67.5, 70 |
| 2001 | 70–89 | 79 | 70.6, 77, 77.1, 79.8, 83.1, 89 |
| 2002 | 62–93 | 78 | 61.9, 77, 80, 93 |
| 2003 | 75–86 | 80 | 75, 77.4, 82.3, 86 |
| 2004 | 79.8–87 | 84 | 79.8, 82.4, 85, 87 |
| 2005 | 85–90 | 87.5 | 85, 90 |
| 2006 | 83–93 | 87.5 | 83, 87, 90, 93 |