Year	Percent
1971	70.13
1972	69.23
1973	67.67
1974	67.43
1975	66.67
1976	65.62
1977	64.18
1978	62.83
1979	63.38
1980	65.58
1981	67.58
1982	67.88
1983	67.44
1984	66.95
1985	66.52
1986	60.78
1987	54.08
1988	50.32
1989	50.07
1990	49.51
1991	47.8
1992	47.38
1993	46.91
1994	46.54
1995	45.53
1996	45.87
1997	45.58
1998	44.56
1999	43.14
2000	43.41
2001	43.38
2002	43.03
2003	43.81
2004	44.44
2005	43.44
2006	44.78
2007	31.52
2008	33.8
2009	28.94
2010	29.72