Year	Percent
1971	60.68
1972	59.95
1973	61.41
1974	62.02
1975	62.79
1976	58.06
1977	63.51
1978	61.82
1979	62.97
1980	61.48
1981	60.71
1982	57.5
1983	58.19
1984	61.16
1985	60.55
1986	64.02
1987	63.05
1988	65.23
1989	64.65
1990	66.74
1991	67.11
1992	68.83
1993	69.34
1994	72.06
1995	73.53
1996	75.16
1997	79.16
1998	78.5
1999	81.08
2000	72.85
2001	68.92
2002	73.53
2003	67.28
2004	63.91
2005	64.89
2006	61.19
2007	60.09
2008	55.57
2009	54.66
2010	52.22