Year	Percent
1970	55.76
1971	55.76
1972	56.9
1973	57.11
1974	57.93
1975	59.66
1976	60.29
1977	60.89
1978	62.53
1979	62.22
1980	62.42
1981	62.73
1982	63.68
1983	64.81
1984	65.53
1985	66.9
1986	67.65
1987	68.36
1988	68.5
1989	68.64
1990	69.39
1991	70.72
1992	71.16
1993	71.1
1994	71.47
1995	72.21
1996	72.55
1997	72.77
1998	73.34
1999	73.72
2000	73.53
2001	74.51
2002	74.7
2003	75.2
2004	75.02
2005	75.29
2006	75.13
2007	75.29
2008	75.07
2009	75.62
2010	75.79
2011	76.35
2012	76.5
2013	76.93
2014	76.15
2015	77.21
2016	77.3
2017	77.98