Year	Percent
1970	61.04
1971	61.62
1972	62.35
1973	62.35
1974	62.69
1975	64.63
1987	70.54
1988	70.52
1989	70.6
1990	71.35
1991	72.63
1992	73.37
1993	74.01
1994	73.98
1995	73.92
1996	74.14
1997	73.94
1998	73.89
1999	74.07
2000	74.19
2001	74.78
2002	74.65
2003	74.92
2004	74.93
2005	75.19
2006	75.78
2007	76.33
2008	76.6
2009	77.76
2010	78.04
2011	77.93
2012	77.78
2013	77.9
2014	78.11
2015	78.43
2016	78.54
2017	78.95