Year Percent 1980 30.5 1981 29.4 1982 30 1983 30.3 1984 29.3 1985 30.1 1986 29.6 1987 29.4 1988 28.7 1989 28.6 1990 29.4 1991 28.3 1992 27.8 1993 29.9 1994 31.2 1995 33.5 1996 34 1997 36.5 1998 35.1 1999 34.6 2000 31.4 2001 29.5 2002 26.7 2003 24.4 2004 25 2005 23.2 2006 21.6 2007 21.6 2008 20.4 2009 20.1