Year	Percent
1970	53.84
1971	55.03
1972	56.14
1973	56.41
1974	56.61
1975	57.36
1976	58.32
1977	59.58
1978	60.9
1979	61.65
1980	62.19
1981	63.1
1982	64.14
1983	64.65
1984	65.1
1985	65.27
1986	66.5
1987	66.01
1988	66.56
1989	66.77
1995	71.01
1996	70.8
1997	71.11
1998	70.99
1999	71.96
2000	72.68
2001	73.45
2002	74.39
2003	74.77
2004	75.1
2005	75.43
2006	75.64
2007	75.83
2008	76.22
2009	77.41
2010	77.8
2011	77.74
2012	77.89
2013	78.37
2014	78.98
2015	79.16
2016	79.4
2017	79.61