| Year | 2001 | 2002 | 2003 | 2004 | 2005 | 2006 | 2007 | 2008 | 2009 | 2010 | 2011 |
| Number of occurrence | 6272 | 6567 | 6126 | 7421 | 12027 | 10154 | 10727 | 9533 | 8246 | 9740 | 9947 |