Year	Percent
1970	58.36
1971	59.19
1972	61.61
1973	61.93
1974	61.68
1975	64.5
1976	66.52
1977	67
1978	67.12
1979	68.05
1980	68.29
1981	68.87
1982	69.96
1983	69.49
1984	70.12
1985	70.49
1986	70.86
1987	71.25
1988	71.32
1989	71.01
1990	71.88
1991	72.42
1992	72.54
1993	72.71
1994	73.32
1995	74.49
1996	75.7
1997	75.46
1998	75.79
1999	75.74
2000	75.9
2001	76.46
2002	78.21
2003	78.35
2004	78.55
2005	79.04
2006	79.09
2007	79.69
2008	80.4
2009	84.12
2010	84.66
2011	84.92