Year	Percent
1971	61.69
1972	61.32
1973	57.97
1974	59.56
1975	62.1
1976	59.24
1977	57.34
1978	53.78
1979	52.53
1980	48.58
1981	49.51
1982	49.19
1983	49.14
1984	49.8
1985	49.84
1986	54.24
1987	49.89
1988	48.54
1989	46.25
1990	46.98
1991	48.27
1992	42.18
1993	42.86
1994	38.12
1995	35.77
1996	35.72
1997	35.14
1998	32.53
1999	35.01
2000	37.08
2001	38.31
2002	37.55
2003	37.32
2004	36.86
2005	37.49
2006	34.32
2007	33.61
2008	34.86
2009	34.65
2010	30.31