Year	Percent
1971	55.5
1972	53.66
1973	55.8
1974	54.4
1975	53.23
1976	52.15
1977	54.01
1978	53.26
1979	53.66
1980	53.94
1981	55.59
1982	57.06
1983	57.43
1984	61.77
1985	62.4
1986	61.07
1987	60.31
1988	61.54
1989	61.61
1990	64.24
1991	64.8
1992	66.25
1993	67.08
1994	67.6
1995	65.87
1996	66.11
1997	65.24
1998	66.71
1999	56.41
2000	57.93
2001	58.33
2002	59.78
2003	64.26
2004	69.78
2005	72.67
2006	70.02
2007	70.21
2008	69.11
2009	68.54
2010	71.15