Year	Percent
1970	45.96
1971	46.99
1972	47.94
1973	47.03
1974	47.24
1975	49.84
1976	51.49
1977	52.86
1978	54.45
1979	55.63
1980	56.51
1981	57.78
1982	58.88
1983	61.72
1984	62.21
1985	62.87
1986	65.26
1987	65.56
1988	67.24
1989	67.04
1990	66.9
1991	67.57
1992	65.08
1993	70.01
1994	69.53
1995	70.46
1996	74.29
1997	74.22
1998	75.17
1999	75.76
2000	76.62
2001	76.81
2002	77.83
2003	77.78
2004	78.98
2005	80.86
2006	81.4
2007	81.2
2008	82.96
2009	83.67
2010	81.15
2011	82.79
2012	84.13
2013	83.98
2014	85.7
2015	78.89
2016	77.36
2017	73.17