| Time | 1998 | 1999 | 2000 | 2001 | 2002 | 2003 | 2004 | 2005 | 2006 | 2007 | 2008 | 2009 | 2010 |
| Respondents | 4557 | 4708 | 5692 | 5603 | 6739 | 5337 | 6623 | 7528 | 9152 | 9991 | 12131 | 13599 | 13748 |
| Favorable (%) | 44.9 | 46.5 | 50.0 | 54.2 | 55.5 | 56.5 | 54.7 | 56.6 | 55.7 | 55.5 | 55.9 | 56.3 | 57.8 |
| Acceptable (%) | 52.7 | 51.9 | 48.3 | 44.0 | 43.9 | 42.1 | 44.4 | 43.1 | 43.7 | 44.2 | 43.7 | 43.4 | 41.9 |
| Negative (%) | 2.4 | 1.6 | 1.4 | 1.0 | 0.6 | 1.4 | 0.9 | 0.4 | 0.6 | 0.4 | 0.4 | 0.3 | 0.3 |