Year	Percent
1970	47.96
1971	48.67
1972	49.39
1973	49.97
1974	50.62
1975	51.75
1976	52.78
1977	53.63
1978	54.63
1979	55.54
1980	56.16
1981	57.17
1982	58.15
1983	59.16
1984	60.22
1985	61.31
1986	62.19
1987	63.05
1988	63.83
1989	64.36
1990	64.83
1991	65.53
1992	64.6
1993	66.94
1994	67.95
1995	68.13
1996	68.64
1997	68.78
1998	69.24
1999	69.44
2000	69.56
2001	69.88
2002	70.46
2003	71.05
2004	71.67
2005	72.32
2006	72.33
2007	73.1
2008	73.62
2009	74.05
2010	74.48
2011	74.61
2012	74.99
2013	75.01
2014	75.84
2015	75.93
2016	75.84
2017	75.95