| Year | Percent | Year | Percent |
|---|---|---|---|
| 1980 | 30.5 | 1995 | 33.5 |
| 1981 | 29.4 | 1996 | 34.0 |
| 1982 | 30.0 | 1997 | 36.5 |
| 1983 | 30.3 | 1998 | 35.1 |
| 1984 | 29.3 | 1999 | 34.6 |
| 1985 | 30.1 | 2000 | 31.4 |
| 1986 | 29.6 | 2001 | 29.5 |
| 1987 | 29.4 | 2002 | 26.7 |
| 1988 | 28.7 | 2003 | 24.4 |
| 1989 | 28.6 | 2004 | 25.0 |
| 1990 | 29.4 | 2005 | 23.2 |
| 1991 | 28.3 | 2006 | 21.6 |
| 1992 | 27.8 | 2007 | 21.6 |
| 1993 | 29.9 | 2008 | 20.4 |
| 1994 | 31.2 | 2009 | 20.1 |