| Year | Percentage | Year | Percentage | Year | Percentage | Year | Percentage |
| 1995 | 20.8 | 2000 | 21.1 | 2005 | 33.9 | 2010 | 26.7 |
| 1996 | 17.9 | 2001 | 20.4 | 2006 | 36.0 | 2011 | 26.5 |
| 1997 | 19.4 | 2002 | 22.6 | 2007 | 34.9 | 2012 | 24.9 |
| 1998 | 18.3 | 2003 | 26.9 | 2008 | 32.0 | 2013 | 23.3 |
| 1999 | 18.2 | 2004 | 30.8 | 2009 | 24.1 | 2014 | 22.9 |