Year	Percent
1970	57.17
1971	57.29
1972	57.57
1973	57.47
1974	58.45
1975	59.29
1976	59.98
1977	60.54
1978	61.59
1979	61.61
1980	62.08
1981	62.72
1982	63.71
1983	63.72
1984	64.28
1985	64.97
1986	65.23
1987	65.02
1988	66.53
1989	67.44
1990	67.48
1991	67.22
1992	67.32
1993	66.97
1994	67.52
1995	67.74
1996	68.44
1997	69.4
1998	71.23
1999	72.23
2000	73.07
2001	73.97
2002	74.67
2003	75.17
2004	75.07
2005	75.7
2006	75.9
2007	75.65
2008	75.68
2009	77.08
2010	77.27
2011	77.16
2012	78.58
2013	79.56
2014	79.72
2015	79.79
2016	79.78
2017	79.72