Year	Percent
1970	53.86
1971	54.97
1972	56.06
1973	56.73
1974	57.51
1975	59.44
1976	60.82
1977	59.2
1978	61.57
1979	59.28
1980	63.64
1981	63.18
1982	66.29
1983	65.01
1984	66.75
1985	65.92
1986	68.4
1987	66.95
1988	68.4
1989	68.28
1990	68.54
1991	69.66
1992	68.22
1993	70.14
1994	71.11
1995	70.6
1996	68.86
1997	69.12
1998	70.27
1999	70.59
2000	70.4
2001	71.16
2002	68.95
2003	74.39
2004	72.93
2005	72.49
2006	73.17
2007	74.73
2008	74.12
2009	72.82
2010	71.72
2011	71.43
2012	72.02
2013	74.6
2014	75.12
2015	74.94
2016	74.64
2017	73.24