Year	Percent
1971	61.14
1972	60.66
1973	59.89
1974	58.12
1975	67.32
1976	68.11
1977	66.77
1978	65.76
1979	64.24
1980	66.72
1981	69.27
1982	70.43
1983	73.28
1984	72.65
1985	72.75
1986	73.04
1987	70.69
1988	70.01
1989	70.07
1990	68.9
1991	66.46
1992	65.97
1993	64.44
1994	62.8
1995	59.32
1996	60.03
1997	58.77
1998	56.25
1999	59.12
2000	58.77
2001	57.44
2002	56.88
2003	59.72
2004	56.72
2005	55.2
2006	57.22
2007	56.77
2008	60.02
2009	57.56
2010	64.97