Year	Percent
1970	32.07
1971	33.67
1972	31.04
1973	30.01
1974	29.82
1975	30.38
1976	29.03
1977	30.67
1978	32.13
1979	34.09
1980	37.02
1981	38.02
1982	40.09
1983	41.14
1984	42.16
1985	44.26
1986	44.51
1987	44.07
1988	44.48
1989	45.27
1990	46.73
1991	47.68
1992	50.22
1993	52.51
1994	53.81
1995	54.79
1996	56.22
1997	57.8
1998	59.99
1999	61.15
2000	61.25
2001	62.57
2002	63.35
2003	63.58
2004	64.43
2005	65.21
2006	65.96
2007	66.72
2008	67.33
2009	68.5
2010	68.38
2011	68.78
2012	69.29
2013	69.5
2014	69.65
2015	69.69
2016	70.19
2017	70.14